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<concept id="GUID-006C503D-1E52-450D-A4DA-8C19B141E09F" xml:lang="en"><title>Introduction

to Intermixing C and C++</title><shortdesc>When porting an open source application or any C (or C++) applications

on top of Symbian/S60 using P.I.P.S., the developer will come across situations

where C and C++ (and Symbian C++ as well) codes will be used together. The

open source community implements a vast number of libraries that export C

APIs to the user of such libraries.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody>

<section id="GUID-88635D46-AEF6-4E8E-969D-D3E56941F289-GENID-1-7-1-11-1-1-5-1-3-1-7-1-4-1-4-1-3-1">       <title>When

and why to use C linkage</title>       <p>While porting such applications,

if developers have to intermix C and C++ code, they then have to depend on

C++ language features like extern "C" for giving C linkage to some set of

APIs. This is required because C and C++ compilers handle function prototype

in a different way. C++ compilers use name mangling, (or name decoration)

to generate unique names for identifiers in a program. It is because of this

technique, in C++, that it is possible to overload any functions (this requires

that all the overloaded functions should have a different argument list).

The C++ mangled name for any identifier contains all the necessary information

that may be needed by the linker, such as linkage type, scope, calling convention,

and others.</p><p>All identifiers (function names or variable names) declared

or defined in C++ source or headers are subject to name mangling. When it

comes to C, however, there is nothing called name mangling or overloading.

It applies when the developer tries to use C APIs by including corresponding

headers. To avoid name mangling, the developer should mention explicitly that

those APIs are C APIs, by using the extern "C" keyword.  </p>     </section>

<section id="GUID-88635D46-AEF6-4E8E-969D-D3E56941F289-GENID-1-7-1-11-1-1-5-1-3-1-7-1-4-1-4-1-3-2">       <title>Syntax

of extern C</title>       <p>The syntax of extern "C" is shown below: </p><codeblock xml:space="preserve">extern "C" declaration ;

</codeblock><p>The declaration (or definition) that immediately follows extern

"C" has the C linkage. </p><codeblock xml:space="preserve">extern "C" { 

   declaration ; 

   declaration ; 

   ... 

}</codeblock><p>Everything between the curly braces has C linkage, unless

declared otherwise. </p>     </section>

<section id="GUID-88635D46-AEF6-4E8E-969D-D3E56941F289-GENID-1-7-1-11-1-1-5-1-3-1-7-1-4-1-4-1-3-3">       <title>How

to use extern C</title>       <p>While writing header files with C functions

which will be included by both C and C++ source files, the user must use extern

"C" properly. See the example below: </p><codeblock xml:space="preserve">/*File: GoodCHeader.h */

/* Can be used by C/C++ header and source files directly */

#ifdef __cplusplus

extern "C" {

#endif

/* Write C function declarations here */

void Function1(int, int);

char* Function3(char*, int);

int Function2(int);

#ifdef __cplusplus

} /* extern "C" */

#endif

</codeblock><p>Using <codeph>ifdef __cplusplus</codeph> is required since

C does not support or recognize the <codeph>extern</codeph> keyword. It is

C++ that provides the mechanisms for mixing C and C++ codes in the same program.

To be precise, C++ supports mixing codes that are compiled by C and C++ compatible

compilers in the same program.</p><p>If the C header is already defined and

it does not have all the APIs defined under extern "C", then while including

such a header in C++ source or header files, extern "C" should be used as

in the example below:  </p><codeblock xml:space="preserve">*File: PureCHeader.h */

/* If C++ header/source files need to include this header, extern "C" should be used*/

/* Define all C APIs here*/

void Function1(int, int);

char* Function3(char*, int);

int Function2(int);

#endif

//File: CSource.cpp

// C++ source file using PureCHeader.h

extern "C" {

#include "PureCHeader.h"

}

void Foo() {

   // use those C APIs here

   int ret = Function2(10);

}

</codeblock>     </section>

<section id="GUID-88635D46-AEF6-4E8E-969D-D3E56941F289-GENID-1-7-1-11-1-1-5-1-3-1-7-1-4-1-4-1-3-4">       <title>Mixing

C and C++ features using extern "C"</title>       <p>The developer

can use all the features of C++ except templates within C by giving those

functions extern "C" linkage. See the example below:</p><codeblock xml:space="preserve">#include &lt;iostream&gt;

using namespace std;

extern "C" {

//Can give C linkage to this class!!

class Sample{

public:

   Sample(int a = 10) : iMem(a) { }

   void Display() { cout<<"iMem is : "<<iMemend1; }

private:

   int iMem;

};

/*

//Can not  give C linkage to template!!

//If we uncomment this code, then it will give a compilation error.

template<class DataType> void Foo(DataType data) {

}*/

}</codeblock>     </section>

<section id="GUID-9AC33F43-5B75-4994-899F-02CD3CE43E5C"><title>Extending C

codes with C++ Codes</title><p>Using C linkage, C codes like structures and

functions that manipulate those structures can be extended efficiently in

a C++ source file as in the example below:  </p><codeblock xml:space="preserve">/* File: CHeader.h */

typedef struct{

   char* name;

   int id;

   float price;

} Item;

int Push(Item* item);

int Pop(Item* item);

/* File: CppHeader.h */

//Give external linkage to C structure

extern "C" {

# include "CHeader.h"

}

class MyStack : public Item {

public:

   MyStack(char* aName, int aId, float aPrice) : name(aName), id(aId), price(aPrice) { }

   int PushOnToStack() { return Push(this);}

   int PopFromStack() { return Pop(this); }

};

</codeblock></section>