View source for Compatibility of C and C++

{{Short description|Comparison of programming languages}}

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The [[C (programming language)|C]] and [[C++]] [[programming language]]s are closely related but have many significant differences. C++ began as a [[Fork (software development)|fork]] of an early, pre-[[ANSI C|standardized]] C, and was designed to be mostly source-and-link compatible with C compilers of the time.<ref name="overview">{{cite web | url=http://www.stroustrup.com/crc.pdf | title=An Overview of the C++ Programming Language in The Handbook of Object Technology (Editor: Saba Zamir). CRC Press LLC, Boca Raton. 1999. ISBN 0-8493-3135-8. | first=Bjarne | last=Stroustrup | author-link=Bjarne Stroustrup | page=4 | access-date=12 August 2009 | url-status=live | archive-url=https://web.archive.org/web/20120816122304/http://www.stroustrup.com/crc.pdf | archive-date=16 August 2012 | df=dmy-all }}</ref><ref>{{cite web |url=http://www.stroustrup.com/siblings_short.pdf |title=C and C++: Siblings. The C/C++ Users Journal. July 2002.|author=B.Stroustrup |access-date=17 March 2019}}</ref>  Due to this, development tools for the two languages (such as [[integrated development environment|IDEs]] and [[compiler]]s) are often integrated into a single product, with the programmer able to specify C or C++ as their source language. 

However, C is ''not'' a [[subset]] of C++,<ref name="subset">{{cite web |url=http://www.stroustrup.com/bs_faq.html#C-is-subset |title=Bjarne Stroustrup's FAQ&nbsp;– Is C a subset of C++? |access-date=22 Sep 2019}}</ref> and nontrivial C programs will not compile as C++ code without modification. Likewise, C++ introduces many features that are not available in C and in practice almost all code written in C++ is not conforming C code. This article, however, focuses on differences that cause conforming C code to be ill-formed C++ code, or to be conforming/well-formed in both languages but to behave differently in C and C++.

[[Bjarne Stroustrup]], the creator of C++, has suggested<ref>{{cite web |url=http://www.stroustrup.com/compat_short.pdf |title=C and C++: A Case for Compatibility. The C/C++ Users Journal. August 2002.|author=B. Stroustrup |access-date=18 August 2013 |url-status=live |archive-url=https://web.archive.org/web/20120722012742/http://www2.research.att.com/~bs/compat_short.pdf |archive-date=22 July 2012 |df=dmy-all }}</ref> that the incompatibilities between C and C++ should be reduced as much as possible in order to maximize interoperability between the two languages. Others have argued that since C and C++ are two different languages, compatibility between them is useful but not vital; according to this camp, efforts to reduce incompatibility should not hinder attempts to improve each language in isolation. The official rationale for the 1999 C standard ([[C99]]) "endorse<nowiki>[d]</nowiki> the principle of maintaining the largest common subset" between C and C++ "while maintaining a distinction between them and allowing them to evolve separately", and stated that the authors were "content to let C++ be the big and ambitious language."<ref>[http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf Rationale for International Standard—Programming Languages—C] {{webarchive|url=https://web.archive.org/web/20160606072228/http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf |date=6 June 2016 }}, revision 5.10 (April 2003).</ref>

Several additions of C99 are not supported in the current C++ standard or conflicted with C++ features, such as [[variable-length array]]s, native [[complex data type|complex number]] types and the <code>[[restrict]]</code> [[type qualifier]]. On the other hand, C99 reduced some other incompatibilities compared with C89 by incorporating C++ features such as <code>//</code> comments and mixed declarations and code.<ref>{{cite web|url=https://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html|title=C Dialect Options - Using the GNU Compiler Collection (GCC)|work=gnu.org|url-status=live|archive-url=https://web.archive.org/web/20140326130603/http://gcc.gnu.org/onlinedocs/gcc/C-Dialect-Options.html|archive-date=26 March 2014|df=dmy-all}}</ref>

==Constructs valid in C but not in C++==
C++ enforces stricter typing rules (no implicit violations of the static type system<ref name="overview" />), and initialization requirements (compile-time enforcement that in-scope variables do not have initialization subverted)<ref>{{cite web|url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf|title=N4659: Working Draft, Standard for Programming Language C++|at=§Annex C.1|url-status=live|archive-url=https://web.archive.org/web/20171207092618/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf|archive-date=7 December 2017|df=dmy-all}} ("It is invalid to jump past a declaration with explicit or implicit initializer (except across entire block not entered). … With this simple compile-time rule, C++ assures that if an initialized variable is in scope, then it has assuredly been initialized.")</ref> than C, and so some valid C code is invalid in C++. A rationale for these is provided in Annex C.1 of the ISO C++ standard.<ref>{{cite web|url=http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf|title=N4659: Working Draft, Standard for Programming Language C++|at=§Annex C.1|url-status=live|archive-url=https://web.archive.org/web/20171207092618/http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf|archive-date=7 December 2017|df=dmy-all}}</ref>

{{bulleted list
|1= One commonly encountered difference is C being more [[weakly-typed]] regarding pointers. Specifically, C allows a <code>void*</code> pointer to be assigned to any pointer type without a cast, while C++ does not; this [[Programming idiom|idiom]] appears often in C code using <code>malloc</code> memory allocation,<ref name="publib.boulder.ibm.com">{{cite web|url=http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=%2Fcom.ibm.vacpp6m.doc%2Flanguage%2Fref%2Fclrc07cplr243.htm|title=IBM Knowledge Center|work=ibm.com}}</ref> or in the passing of context pointers to the POSIX [[pthreads]] API, and other frameworks involving [[callbacks]].  For example, the following is valid in C but not C++:

<syntaxhighlight lang="c">void *ptr;
/* Implicit conversion from void* to int* */
int *i = ptr;
</syntaxhighlight>

or similarly:

<syntaxhighlight lang="c">int *j = malloc(5 * sizeof *j);     /* Implicit conversion from void* to int* */</syntaxhighlight>

In order to make the code compile as both C and C++, one must use an explicit cast, as follows (with some caveats in both languages):<ref>{{cite web|url=http://faq.cprogramming.com/cgi-bin/smartfaq.cgi?id=1043284351&answer=1047673478|title=FAQ > Casting malloc - Cprogramming.com|website=faq.cprogramming.com|url-status=live|archive-url=https://web.archive.org/web/20070405121807/http://faq.cprogramming.com/cgi-bin/smartfaq.cgi?answer=1047673478&id=1043284351|archive-date=5 April 2007|df=dmy-all}}</ref>

<syntaxhighlight lang="c++">
void *ptr;
int *i = (int *)ptr;
int *j = (int *)malloc(5 * sizeof *j);
</syntaxhighlight>

|2= C++ has more complicated rules about pointer assignments that add qualifiers as it allows the assignment of <code>int **</code>
to <code>const int *const *</code> but not the unsafe assignment to <code>const int **</code> while C allows neither of those (although compilers will usually only emit a warning).

|3= C++ changes some [[C standard library]] functions to add additional overloaded functions with <code>[[const]]</code> [[type qualifier]]s, e.g. <code>strchr</code> returns <code>char*</code> in C, while C++ acts as if there were two overloaded functions <code>const char *strchr(const char *)</code> and a <code>char *strchr(char *)</code>. In [[C23 (C standard revision)|C23]] generic selection is used to make C's behaviour more similar to C++'s.<ref>{{cite web|url=https://www.open-std.org/jtc1/sc22/wg14/www/docs/n3020.pdf|title=Qualifier-preserving standard library functions, v4}}</ref>

|4= C++ is also more strict in conversions to enums: ints cannot be implicitly converted to enums as in C. Also, [[enumerated type#C and syntactically similar languages|enumeration constants]] (<code>enum</code> enumerators) are always of type <code>int</code> in C, whereas they are distinct types in C++ and may have a size different from that of <code>int</code>.{{update inline|reason=In C23 the underlying type of an enum can be different from int. See [https://open-std.org/JTC1/SC22/WG14/www/docs/n3029.htm N3029] and [https://open-std.org/JTC1/SC22/WG14/www/docs/n3030.htm N3030].|date=February 2024}}

|5= In C++ a <code>const</code> variable must be initialized; in C this is not necessary.

|6= C++ compilers prohibit goto or switch from crossing an initialization, as in the following C99 code:
<syntaxhighlight lang="c">
void fn(void)
{
    goto flack;
    int i = 1;
flack:
    ;
}
</syntaxhighlight>

|7= While syntactically valid, a [[longjmp|<code>longjmp()</code>]] results in undefined behaviour in C++ if the jumped-over [[stack frame]]s include objects with nontrivial destructors.<ref>{{cite web|url=http://www.cplusplus.com/reference/csetjmp/longjmp/|title=longjmp - C++ Reference|website=www.cplusplus.com|url-status=live|archive-url=https://web.archive.org/web/20180519163512/http://www.cplusplus.com/reference/csetjmp/longjmp/|archive-date=19 May 2018|df=dmy-all}}</ref> The C++ implementation is free to define the behaviour such that destructors would be called. However, this would preclude some uses of <code>longjmp()</code> which would otherwise be valid, such as implementation of [[Thread (computing)|threads]] or [[coroutine]]s switching between separate call stacks with <code>longjmp()</code> — when jumping from the lower to the upper call stack in global address space, destructors would be called for every object in the lower call stack. No such issue exists in C.

|8= C allows for multiple tentative definitions of a single global variable in a single [[Translation unit (programming)|translation unit]], which is invalid as an [[One Definition Rule|ODR]] violation in C++.
<syntaxhighlight lang="c">
int N;
int N = 10;
</syntaxhighlight>

|9= In C, declaring a new type with the same name as an existing <code>struct</code>, <code>union</code> or <code>enum</code> is valid, but it is invalid in C++, because in C, <code>struct</code>, <code>union</code>, and <code>enum</code> types must be indicated as such whenever the type is referenced whereas in C++, all declarations of such types carry the [[typedef]] implicitly.
<syntaxhighlight lang="c">
enum BOOL {FALSE, TRUE};
typedef int BOOL;
</syntaxhighlight>

|10= Non-prototype ("K&amp;R"-style) function declarations are invalid in C++; they are still valid in C until C23,<ref name=N2432>{{cite web|url=https://open-std.org/JTC1/SC22/WG14/www/docs/n2432.pdf|title=WG14 N2432 : Remove support for function definitions with identifier lists}}</ref><ref>{{cite web|url=http://www.open-std.org/JTC1/sc22/wg14/www/docs/n1570.pdf|title=2011 ISO C draft standard}}</ref> although they have been deemed obsolescent since C's original standardization in 1990. (The term "obsolescent" is a defined term in the ISO C standard, meaning a feature that "may be considered for withdrawal in future revisions" of the standard.) Similarly, implicit function declarations (using functions that have not been declared) are not allowed in C++, and have been invalid in C since 1999.

|11= In C until C23,<ref name=N2841>{{cite web|url=https://open-std.org/JTC1/SC22/WG14/www/docs/n2841.htm|title= WG14 N 2841: No function declarators without prototypes}}</ref> a function declaration without parameters, e.g. <code>int foo();</code>, implies that the parameters are unspecified. Therefore, it is legal to call such a function with one or more [[argument (computer programming)|argument]]s, e.g. <code>foo(42, "hello world")</code>. In contrast, in C++ a function prototype without arguments means that the function takes no arguments, and calling such a function with arguments is ill-formed. In C, the correct way to declare a function that takes no arguments is by using 'void', as in <code>int foo(void);</code>, which is also valid in C++. Empty function prototypes are a deprecated feature in C99 (as they were in C89).

|12= In both C and C++, one can define nested <code>[[struct (C programming language)|struct]]</code> types, but the scope is interpreted differently: in C++, a nested <code>struct</code> is defined only within the scope/namespace of the outer <code>struct</code>, whereas in C the inner struct is also defined outside the outer struct.

|13= C allows <code>[[struct (C programming language)|struct]]</code>, <code>[[Union (computer science)|union]]</code>, and <code>[[Enumerated type#C and syntactically similar languages|enum]]</code> types to be declared in function prototypes, whereas C++ does not.
}}

[[C99]] and [[C11 (C standard revision)|C11]] added several additional features to C that have not been incorporated into standard C++ as of [[C++20]], such as complex numbers, variable length arrays (complex numbers and variable length arrays are designated as optional extensions in C11), [[flexible array member]]s, the [[restrict]] keyword, array parameter qualifiers, and [[C syntax#Compound literals|compound literals]]. 

{{bulleted list
|1= [[Complex arithmetic]] using the <code>float complex</code> and <code>double complex</code> primitive data types was added in the [[C99]] standard, via the <code>_Complex</code> keyword and <code>complex</code> convenience macro. In C++, complex arithmetic can be performed using the complex number class, but the two methods are not code-compatible. (The standards since [[C++11]] require binary compatibility, however.)<ref>{{cite web|url=http://en.cppreference.com/w/cpp/numeric/complex|title=std::complex - cppreference.com|website=en.cppreference.com|url-status=live|archive-url=https://web.archive.org/web/20170715125445/http://en.cppreference.com/w/cpp/numeric/complex|archive-date=15 July 2017|df=dmy-all}}</ref>

|2= Variable length arrays. This feature leads to possibly non-compile time {{mono|[[sizeof]]}} operator.<ref>{{cite web|title=Incompatibilities Between ISO C and ISO C++|url=http://david.tribble.com/text/cdiffs.htm#C90-vs-CPP98|url-status=live|archive-url=https://web.archive.org/web/20060409075755/http://david.tribble.com/text/cdiffs.htm#C90-vs-CPP98|archive-date=9 April 2006|df=dmy-all}}</ref>
<syntaxhighlight lang="C">
void foo(size_t x, int a[*]);  // VLA declaration
void foo(size_t x, int a[x]) 
{
    printf("%zu\n", sizeof a); // same as sizeof(int*)
    char s[x * 2];
    printf("%zu\n", sizeof s); // will print x*2
}
</syntaxhighlight>
<!-- * C99 allows declaration of function macros with empty arguments. For example macro <code>#define ADD(A,B) (A+B)</code> applied to <code>ADD(4,)</code> will be evaluated to <code>(4+)</code> -->

|3= The last member of a C99 structure type with more than one member may be a [[flexible array member]], which takes the syntactic form of an array with unspecified length.  This serves a purpose similar to variable-length arrays, but VLAs cannot appear in type definitions, and unlike VLAs, flexible array members have no defined size.  ISO C++ has no such feature.  Example:
<syntaxhighlight lang="C">
struct X
{
    int n, m;
    char bytes[];
}
</syntaxhighlight>

|4= The <code>[[restrict]]</code> [[type qualifier]] defined in C99 was not included in the C++03 standard, but most mainstream compilers such as the [[GNU Compiler Collection]],<ref>[https://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html#Restricted-Pointers Restricted Pointers] {{webarchive|url=https://web.archive.org/web/20160806092010/http://gcc.gnu.org/onlinedocs/gcc/Restricted-Pointers.html |date=6 August 2016 }} from ''Using the GNU Compiler Collection (GCC)''</ref> [[Microsoft Visual C++]], and [[Intel C++ Compiler]] provide similar functionality as an extension.

|5= Array parameter qualifiers in functions are supported in C but not C++.
<syntaxhighlight lang="C">
int foo(int a[const]);     // equivalent to int *const a 
int bar(char s[static 5]); // annotates that s is at least 5 chars long
</syntaxhighlight>

|6= The functionality of [[C syntax#Compound literals|compound literals]] in C is generalized to both built-in and user-defined types by the list initialization syntax of C++11, although with some syntactic and semantic differences. 
<syntaxhighlight lang="C">
struct X a = (struct X){4, 6};  // The equivalent in C++ would be X{4, 6}. The C syntactic form used in C99 is supported as an extension in the GCC and Clang C++ compilers.
foo(&(struct X){4, 6});         // The object is allocated in the stack and its address can be passed to a function. This is not supported in C++.

if (memcmp(d, (int []){8, 6, 7, 5, 3, 0, 9}, n) == 0) {} // The equivalent in C++ would be using digits = int []; if (memcmp(d, digits{8, 6, 7, 5, 3, 0, 9}, n) == 0) {}
</syntaxhighlight>

|7= [[C syntax#Designated initializers|Designated initializers]] for arrays are valid only in C:
<syntaxhighlight lang="C">
char s[20] = { [0] = 'a', [8] = 'g' };  // allowed in C, not in C++
</syntaxhighlight>

|8= Functions that do not return can be annotated using a <code>noreturn</code> [[C++11#Attributes|attribute]] in C++ whereas C uses a distinct keyword. In C23, the attribute syntax is also supported.<ref name="N2764">{{cite web |title=WG14-N2764 : The noreturn attribute |url=https://open-std.org/JTC1/SC22/WG14/www/docs/n2764.pdf |website=open-std.org |archive-url=https://web.archive.org/web/20221225063437/https://open-std.org/JTC1/SC22/WG14/www/docs/n2764.pdf |archive-date=December 25, 2022 |date=2021-06-21 |url-status=live}}</ref>
}}

C++ adds numerous additional keywords to support its new features. This renders C code using those keywords for identifiers invalid in C++. For example:

<syntaxhighlight lang="c">
struct template 
{
    int new;
    struct template* class;
};
</syntaxhighlight>

:is valid C code, but is rejected by a C++ compiler, since the keywords <code>template</code>, <code>new</code> and <code>class</code> are reserved.

==Constructs that behave differently in C and C++==
There are a few syntactic constructs that are valid in both C and C++ but produce different results in the two languages.

* [[Character literal]]s such as <code>'a'</code> are of type <code>int</code> in C and of type <code>char</code> in C++, which means that <code>sizeof 'a'</code><!--Thought this was an error for sizeof('a') but both seems to work--> will generally give different results in the two languages: in C++, it will be <code>1</code>, while in C it will be <code>sizeof(int)</code>. As another consequence of this type difference, in C, <code>'a'</code> will always be a signed expression, regardless of whether or not <code>char</code> is a signed or unsigned type, whereas for C++ this is compiler implementation specific.
* C++ assigns internal linkage to namespace-scoped <code>const</code> variables unless they are explicitly declared <code>[[extern]]</code>, unlike C in which <code>extern</code> is the default for all file-scoped entities. In practice this does not lead to silent semantic changes between identical C and C++ code but instead will lead to a compile-time or linkage error.
* In C, use of inline functions requires manually adding a prototype declaration of the function using the extern keyword in exactly one translation unit to ensure a non-inlined version is linked in, whereas C++ handles this automatically. In more detail, C distinguishes two kinds of definitions of [[Inline function|<code>inline</code> functions]]: ordinary external definitions (where <code>extern</code> is explicitly used) and inline definitions.  C++, on the other hand, provides only inline definitions for inline functions.  In C, an inline definition is similar to an internal (i.e. static) one, in that it can coexist in the same program with one external definition and any number of internal and inline definitions of the same function in other translation units, all of which can differ.  This is a separate consideration from the ''linkage'' of the function, but not an independent one.  C compilers are afforded the discretion to choose between using inline and external definitions of the same function when both are visible.  C++, however, requires that if a function with external linkage is declared <code>inline</code> in any translation unit then it must be so declared (and therefore also defined) in every translation unit where it is used, and that all the definitions of that function be identical, following the ODR. Static inline functions behave identically in C and C++.
* Both C (since C99) and C++ have a [[Boolean type]] <code>bool</code> with constants <code>true</code> and <code>false</code>, but they are defined differently.
** In C++, <code>bool</code> is a [[built-in type]] and a [[reserved keyword]].
** In C99, a new keyword, <code>_Bool</code>, is introduced as a new built-in Boolean type. The header <code>stdbool.h</code> provides macros <code>bool</code>, <code>true</code> and <code>false</code> that are defined as <code>_Bool</code>, <code>1</code> and <code>0</code>, respectively. Therefore, <code>true</code> and <code>false</code> have the type <code>int</code>.
** In [[C23 (C standard revision)|C23]] however, <code>bool</code>, <code>true</code>, and <code>false</code> are keywords, with <code>true</code> and <code>false</code> having the type <code>bool</code>.
* C++ has the types <code>char8_t</code>, <code>char16_t</code> and <code>char32_t</code> to encode a single [[Unicode Transformation Format|UTF]] code unit. C23 includes these, but as typedefs to other integer types rather than distinct built-in types, such that their names are not reserved keywords.
* In C it is implementation-defined whether a [[bit field]] of type <code>int</code> is signed or unsigned while in C++ it is always signed to match the underlying type.

Several of the other differences from the previous section can also be exploited to create code that compiles in both languages but behaves differently.  For example, the following function will return different values in C and C++:

<syntaxhighlight lang="C">
extern int T;

int size(void)
{
    struct T {  int i;  int j;  };
    
    return sizeof(T);
    /* C:   return sizeof(int)
     * C++: return sizeof(struct T)
     */
}
</syntaxhighlight>

This is due to C requiring <code>struct</code> in front of structure tags (and so <code>sizeof(T)</code> refers to the variable), but C++ allowing it to be omitted (and so <code>sizeof(T)</code> refers to the implicit <code>typedef</code>). Beware that the outcome is different when the <code>extern</code> declaration is placed inside the function: then the presence of an identifier with same name in the function scope inhibits the implicit <code>typedef</code> to take effect for C++, and the outcome for C and C++ would be the same. Observe also that the ambiguity in the example above is due to the use of the parenthesis with the <code>sizeof</code> operator. Using <code>sizeof T</code> would expect <code>T</code> to be an expression and not a type, and thus the example would not compile with C++.

==Linking C and C++ code==
While C and C++ maintain a large degree of source compatibility, the object files their respective compilers produce can have important differences that manifest themselves when intermixing C and C++ code.  Notably:
* C compilers do not [[name mangling|name mangle]] symbols in the way that C++ compilers do.<ref>{{cite web|url=http://publib.boulder.ibm.com/infocenter/macxhelp/v6v81/index.jsp?topic=%2Fcom.ibm.vacpp6m.doc%2Flanguage%2Fref%2Fclrc01name_mangling.htm|title=IBM Knowledge Center|work=ibm.com}}</ref>
* Depending on the compiler and architecture, it also may be the case that [[calling convention]]s differ between the two languages.

For these reasons, for C++ code to call a C function <code>foo()</code>, the C++ code must [[Function prototype|prototype]] <code>foo()</code> with <code>extern "C"</code>. Likewise, for C code to call a C++ function <code>bar()</code>, the C++ code for <code>bar()</code> must be declared with <code>extern "C"</code>.

A common practice for [[header file]]s to maintain both C and C++ compatibility is to make its declaration be <code>extern "C"</code> for the scope of the header:<ref>{{cite web|url=http://publib.boulder.ibm.com/infocenter/comphelp/v8v101/index.jsp?topic=%2Fcom.ibm.xlcpp8a.doc%2Fstandlib%2Fref%2Fstdcpplib.htm|title=IBM Knowledge Center|work=ibm.com}}</ref>

<syntaxhighlight lang="c">
/* Header file foo.h */
#ifdef __cplusplus /* If this is a C++ compiler, use C linkage */
extern "C" {
#endif

/* These functions get C linkage */
void foo();
 
struct bar { /* ... */ };

#ifdef __cplusplus /* If this is a C++ compiler, end C linkage */
}
#endif
</syntaxhighlight>

Differences between C and C++ [[Linkage (software)|linkage]] and calling conventions can also have subtle implications for code that uses [[function pointer]]s. Some compilers will produce non-working code if a function pointer declared <code>extern "C"</code> points to a C++ function that is not declared <code>extern "C"</code>.<ref>{{cite web |url=http://docs.sun.com/source/819-3689/Ch3.Std.html#pgfId-18503 |title=Oracle Documentation |publisher=Docs.sun.com |access-date=18 August 2013 |url-status=live |archive-url=https://web.archive.org/web/20090403134627/http://docs.sun.com/source/819-3689/Ch3.Std.html#pgfId-18503 |archive-date=3 April 2009 |df=dmy-all }}</ref>

For example, the following code:

<syntaxhighlight lang="c" line highlight="2,6">
void my_function();
extern "C" void foo(void (*fn_ptr)(void));

void bar()
{
   foo(my_function);
}
</syntaxhighlight>

Using [[Sun Microsystems]]' C++ compiler, this produces the following warning:

<syntaxhighlight lang="c" highlight="1">
 $ CC -c test.cc
 "test.cc", line 6: Warning (Anachronism): Formal argument fn_ptr of type
 extern "C" void(*)() in call to foo(extern "C" void(*)()) is being passed
 void(*)().
</syntaxhighlight>

This is because <code>my_function()</code> is not declared with C linkage and calling conventions, but is being passed to the C function <code>foo()</code>.

==References==
{{Reflist|30em}}

==External links==
{{wikibooks|C++ Programming|Programming Languages/Comparisons/C}}
* [https://www.coding-guidelines.com/cbook/c90c++.pdf Detailed comparison], sentence by sentence, from a C89 Standard perspective.
* [http://david.tribble.com/text/cdiffs.htm  Incompatibilities Between ISO C and ISO C++], David R. Tribble (August 2001).
* [https://docs.oracle.com/cd/E19422-01/819-3689/Ch3.Std.html#pgfId-18041 Oracle (Sun Microsystems) C++ Migration Guide, section 3.11], Oracle/Sun compiler docs on linkage scope.
* [https://www.oracle.com/technetwork/articles/servers-storage-dev/mixingcandcpluspluscode-305840.html Oracle: Mixing C and C++ Code in the Same Program], overview by Steve Clamage (ANSI C++ Committee chair).

{{CProLang}}
{{C++ programming language}}

{{Use dmy dates|date=January 2015}}

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[[Category:C (programming language)]]
[[Category:C++]]
[[Category:Articles with example C++ code]]