C++ linkage error for inline member function

Question

I have the following code:

IFile.h

class IFile
{
public:
    IFile();
    ~IFile(void);

    inline bool IsValidFileType() const;
};

IFile.cpp

IFile::IFile()
{
    //IsValidFileType();
}

IFile::~IFile(void)
{
}

inline bool IFile::IsValidFileType() const
{
          return true;
}

main.cpp

int main(int argc, char* argv[])
{
    IFile* pFile = new IFile();
    pFile->IsValidFileType();

    return 0;
}

When compiling the code I get the following error: error LNK2019: unresolved external symbol "public: bool __thiscall IFile::IsValidFileType(void)const " (?IsValidFileType@IFile@@QBE_NXZ) referenced in function _main

If I change wither "inline" or "const" qualiferes for the function, or call it inside the constructor, the program will complile. Can you please explain this behaviour?

For it to be inlined, it needs to be visible at the call location. Putting it in the header would be one way to fix it — Peter Wood, Mar 01 '13 at 14:43
Please search before asking. There are so many duplicates for this.... — JoergB, Mar 01 '13 at 14:45

score 0 · Answer 1 · answered Mar 01 '13 at 14:44

0

How can the compiler inline a function whose code it cannot see while it is compiling? When compiling main.cpp, the compiler is being asked to do just this.

answered Mar 01 '13 at 14:44

David Schwartz

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score 0 · Answer 2 · answered Mar 01 '13 at 14:44

0

An inline function's code gets compiled into each translation unit that references it (that's the idea, after all). Meaning, you need to include the code in the header file.

answered Mar 01 '13 at 14:44

score 0 · Answer 3 · answered Mar 01 '13 at 14:45

0

The inline keyword promises to the compiler that it will be able to see the definition in each translation unit (*.cpp file) in which it is used. You break this promise, since main.cpp can't see the definition although it includes IFile.h.

Usually functions with the inline keyword should be defined in a header file, not a source file.

answered Mar 01 '13 at 14:45

aschepler

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Thank you. That makes sense, but why removing the const also solves the issue? – user2123914 Mar 01 '13 at 14:46
I can't see any reason removing the `const`s and doing nothing else would make a difference here. – aschepler Mar 01 '13 at 14:52

score 0 · Answer 4 · answered Mar 01 '13 at 14:45

0

Since the function is inline, you have to define it in the header file, not in the cpp file.

answered Mar 01 '13 at 14:45

NPE

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C++ linkage error for inline member function

4 Answers4