Static function declared but not defined in C++

问题

I'm getting an error from the following code using C++.

Main.cpp

#include "file.h"

int main()
{
   int k = GetInteger();
   return 0;
}

File.h

static int GetInteger();

File.cpp

#include "file.h"

static int GetInteger()
{
   return 1;
}

The error I get:

Error C2129: static function 'int GetInteger(void)' declared but not defined.

I've read the famous article "Organizing Code File in C and C++", but don't understand what is wrong with this code.

回答1:

In C++, static at global/namespace scope means the function/variable is only used in the translation unit where it is defined, not in other translation units.

Here you are trying to use a static function from a different translation unit (Main.cpp) than the one in which it is defined (File.cpp).

Remove the static and it should work fine.

回答2:

Change

static int GetInteger();

int GetInteger();

static in this case gives the method internal linkeage, meaning that you can only use it in the translation unit where you define it.

You define it in File.cpp and try to use it in main.cpp, but main doesn't have a definition for it, since you declared it static.

回答3:

Because in this case, static means that the name of the function has internal linkage; that GetInteger in one translation unit is unrelated to GetInteger in any other translation unit. The keyword static is overloaded: in some cases, it affects lifetime, and in others, binding. It's particularly confusing here, because "static" is also the name of a lifetime. Functions, and data declared at namespace scope, always have static lifetime; when static appears in their declaration, it causes internal binding, instead of external.

回答4:

functions declared as static arelocal to the containing file. Therefore, you have to define the function in the same file as the ones who call it. If you want to make it callable from other file, you must NOT declare it as static.

回答5:

From my understanding, static functions are name mangled with the filename in which they are defined so when you include file.h in main.cpp, GetInteger() get mangled with main.cpp though you have defined GetInteger() in file.cpp but since it is static it gets mangled too and linker cannot find the definition of GetInteger() as no function by this name exists.

I believe lesson learnt is don't declare static functions in headerfile as are not intended to be a part of interface.

回答6:

If everything is in the same translation unit it should work. You probably did not compile File.cpp into the same unit as Main.cpp.

g++ -Wall File.cpp Main.cpp

If each file is compiled separately the function must be made extern to be used from a different translation unit.

extern int GetInteger();

which is the same as

int GetInteger();

来源：https://stackoverflow.com/questions/10812769/static-function-declared-but-not-defined-in-c

标签

c++

static-methods