CppLinkErrorMultipleDefinition.md

 

 

 

 

 

(C++) Multiple definition of 'X'

 

Multiple definition of 'X' is a link error.

 

Multiple definition of 'X' is caused when the linker encounters multiple definitions of the same (member) function. For example, see the code below:

 

---

#ifndef A_H #define A_H struct A {   A(); }; #include <iostream> A::A() { std::cout << "Hi\n"; } #endif // A_H

 

This header file contains both a declaration and definition of the constructor of class A.

 

'Huh, but there is an #include guard!', you might think. Correct, this does prevent the compile error 'Redefinition of '. But the (non-inline) definition of the constructor of class A will be present in all translation units that #include this header file.

 

This is an example of data (in this case a member function) with external linkage. Avoid data with external linkage at file scope [1]. Avoid non-inline function definitions in header files [2].

 

Some solutions are:

• Put the declarations in header files, put the definitions in implementation files
• Make the declarations inline
• Ensure the header file is used by exactly one unit

 

The first solution is by far the most common. The latter solution is not always possible.

 

Below is a full project with this link error.

 

• Download the Qt Creator project 'CppLinkErrorMultipleDefinition' (zip)

 

 

 

 

 

Technical facts

 

Application type(s)

• Desktop application

Operating system(s) or programming environment(s)

• Lubuntu 12.10 (quantal)

IDE(s):

• Qt Creator 2.5.2

Project type:

• Console application

C++ standard:

• C++98

Compiler(s):

• G++ 4.7.2

Libraries used:

• STL: GNU ISO C++ Library, version 4.7.2

 

 

 

 

 

Qt project file: CppLinkErrorMultipleDefinition.pro

 

---

TEMPLATE = app CONFIG += console CONFIG -= qt SOURCES += \     main.cpp \     a.cpp HEADERS += \     a.h

 

 

 

 

 

a.h

 

---

#ifndef A_H #define A_H struct A {   A();   void SayHello() const;   inline void SayHelloInline() const; }; #include <iostream> //A::A() { std::cout << "Constructor\n"; } //Illegal, cannot put constructor here //void A::SayHello() const { std::cout << "Hello\n"; } //Illegal, cannot put non-inline definition here inline void A::SayHelloInline() const { std::cout << "HelloInline\n"; } #endif // A_H

 

 

 

 

 

a.cpp

 

---

#include "a.h" A::A() { std::cout << "Constructor\n"; } //Legal, can put constructor here void A::SayHello() const { std::cout << "Hello\n"; } //Legal, can put non-inline definition here

 

 

 

 

 

main.cpp

 

---

#include "a.h" int main() {   A a;   a.SayHello();   a.SayHelloInline(); }

 

 

 

 

 

References

 

1. John Lakos. Large-Scale C++ Software Design. 1996. ISBN: 0-201-63362-0. Chapter 2.3.1, page 70: 'Avoid data with external linkage at file scope'
2. Bjarne Stroustrup. The C++ Programming Language (3rd edition). 1997. ISBN: 0-201-88954-4. Section 9.5, item 4: 'Avoid non-inline function definitions in headers'