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1 <?xml version="1.0" encoding="utf-8"?> |
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2 <!-- Copyright (c) 2007-2010 Nokia Corporation and/or its subsidiary(-ies) All rights reserved. --> |
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3 <!-- This component and the accompanying materials are made available under the terms of the License |
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4 "Eclipse Public License v1.0" which accompanies this distribution, |
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5 and is available at the URL "http://www.eclipse.org/legal/epl-v10.html". --> |
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6 <!-- Initial Contributors: |
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7 Nokia Corporation - initial contribution. |
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8 Contributors: |
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9 --> |
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10 <!DOCTYPE concept |
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11 PUBLIC "-//OASIS//DTD DITA Concept//EN" "concept.dtd"> |
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12 <concept id="GUID-006C503D-1E52-450D-A4DA-8C19B141E09F" xml:lang="en"><title>Introduction |
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13 to Intermixing C and C++</title><shortdesc>When porting an open source application or any C (or C++) applications |
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14 on top of Symbian/S60 using P.I.P.S., the developer will come across situations |
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15 where C and C++ (and Symbian C++ as well) codes will be used together. The |
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16 open source community implements a vast number of libraries that export C |
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17 APIs to the user of such libraries.</shortdesc><prolog><metadata><keywords/></metadata></prolog><conbody> |
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18 <section id="GUID-88635D46-AEF6-4E8E-969D-D3E56941F289-GENID-1-8-1-11-1-1-5-1-3-1-7-1-4-1-4-1-3-1"> <title>When |
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19 and why to use C linkage</title> <p>While porting such applications, |
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20 if developers have to intermix C and C++ code, they then have to depend on |
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21 C++ language features like extern "C" for giving C linkage to some set of |
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22 APIs. This is required because C and C++ compilers handle function prototype |
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23 in a different way. C++ compilers use name mangling, (or name decoration) |
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24 to generate unique names for identifiers in a program. It is because of this |
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25 technique, in C++, that it is possible to overload any functions (this requires |
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26 that all the overloaded functions should have a different argument list). |
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27 The C++ mangled name for any identifier contains all the necessary information |
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28 that may be needed by the linker, such as linkage type, scope, calling convention, |
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29 and others.</p><p>All identifiers (function names or variable names) declared |
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30 or defined in C++ source or headers are subject to name mangling. When it |
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31 comes to C, however, there is nothing called name mangling or overloading. |
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32 It applies when the developer tries to use C APIs by including corresponding |
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33 headers. To avoid name mangling, the developer should mention explicitly that |
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34 those APIs are C APIs, by using the extern "C" keyword. </p> </section> |
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35 <section id="GUID-88635D46-AEF6-4E8E-969D-D3E56941F289-GENID-1-8-1-11-1-1-5-1-3-1-7-1-4-1-4-1-3-2"> <title>Syntax |
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36 of extern C</title> <p>The syntax of extern "C" is shown below: </p><codeblock xml:space="preserve">extern "C" declaration ; |
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37 </codeblock><p>The declaration (or definition) that immediately follows extern |
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38 "C" has the C linkage. </p><codeblock xml:space="preserve">extern "C" { |
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39 declaration ; |
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40 declaration ; |
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41 ... |
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42 }</codeblock><p>Everything between the curly braces has C linkage, unless |
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43 declared otherwise. </p> </section> |
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44 <section id="GUID-88635D46-AEF6-4E8E-969D-D3E56941F289-GENID-1-8-1-11-1-1-5-1-3-1-7-1-4-1-4-1-3-3"> <title>How |
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45 to use extern C</title> <p>While writing header files with C functions |
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46 which will be included by both C and C++ source files, the user must use extern |
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47 "C" properly. See the example below: </p><codeblock xml:space="preserve">/*File: GoodCHeader.h */ |
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48 /* Can be used by C/C++ header and source files directly */ |
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49 |
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50 #ifdef __cplusplus |
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51 extern "C" { |
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52 #endif |
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53 /* Write C function declarations here */ |
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54 void Function1(int, int); |
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55 char* Function3(char*, int); |
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56 int Function2(int); |
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57 #ifdef __cplusplus |
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58 } /* extern "C" */ |
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59 #endif |
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60 </codeblock><p>Using <codeph>ifdef __cplusplus</codeph> is required since |
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61 C does not support or recognize the <codeph>extern</codeph> keyword. It is |
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62 C++ that provides the mechanisms for mixing C and C++ codes in the same program. |
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63 To be precise, C++ supports mixing codes that are compiled by C and C++ compatible |
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64 compilers in the same program.</p><p>If the C header is already defined and |
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65 it does not have all the APIs defined under extern "C", then while including |
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66 such a header in C++ source or header files, extern "C" should be used as |
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67 in the example below: </p><codeblock xml:space="preserve">*File: PureCHeader.h */ |
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68 /* If C++ header/source files need to include this header, extern "C" should be used*/ |
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69 /* Define all C APIs here*/ |
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70 void Function1(int, int); |
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71 char* Function3(char*, int); |
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72 int Function2(int); |
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73 #endif |
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74 |
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75 |
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76 //File: CSource.cpp |
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77 // C++ source file using PureCHeader.h |
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78 extern "C" { |
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79 #include "PureCHeader.h" |
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80 } |
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81 void Foo() { |
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82 // use those C APIs here |
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83 int ret = Function2(10); |
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84 } |
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85 </codeblock> </section> |
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86 <section id="GUID-88635D46-AEF6-4E8E-969D-D3E56941F289-GENID-1-8-1-11-1-1-5-1-3-1-7-1-4-1-4-1-3-4"> <title>Mixing |
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87 C and C++ features using extern "C"</title> <p>The developer |
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88 can use all the features of C++ except templates within C by giving those |
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89 functions extern "C" linkage. See the example below:</p><codeblock xml:space="preserve">#include <iostream> |
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90 using namespace std; |
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91 |
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92 extern "C" { |
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93 |
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94 //Can give C linkage to this class!! |
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95 class Sample{ |
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96 public: |
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97 Sample(int a = 10) : iMem(a) { } |
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98 void Display() { cout<<"iMem is : "<<iMemend1; } |
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99 private: |
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100 int iMem; |
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101 }; |
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102 |
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103 /* |
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104 //Can not give C linkage to template!! |
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105 //If we uncomment this code, then it will give a compilation error. |
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106 template<class DataType> void Foo(DataType data) { |
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107 }*/ |
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108 }</codeblock> </section> |
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109 <section id="GUID-9AC33F43-5B75-4994-899F-02CD3CE43E5C"><title>Extending C |
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110 codes with C++ Codes</title><p>Using C linkage, C codes like structures and |
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111 functions that manipulate those structures can be extended efficiently in |
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112 a C++ source file as in the example below: </p><codeblock xml:space="preserve">/* File: CHeader.h */ |
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113 typedef struct{ |
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114 char* name; |
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115 int id; |
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116 float price; |
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117 } Item; |
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118 |
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119 int Push(Item* item); |
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120 int Pop(Item* item); |
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121 |
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122 |
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123 |
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124 /* File: CppHeader.h */ |
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125 //Give external linkage to C structure |
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126 extern "C" { |
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127 # include "CHeader.h" |
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128 } |
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129 |
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130 class MyStack : public Item { |
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131 public: |
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132 MyStack(char* aName, int aId, float aPrice) : name(aName), id(aId), price(aPrice) { } |
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133 int PushOnToStack() { return Push(this);} |
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134 int PopFromStack() { return Pop(this); } |
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135 }; |
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136 </codeblock></section> |
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137 </conbody></concept> |