c++ - inline function linker error

Question

I am trying to use inline member functions of a particular class. For example the function declaration and implementation without inlining is as such:

in the header file:

int GetTplLSize();

in the .cpp file:

int NeedleUSsim::GetTplLSize()
{
    return sampleDim[1];
}

For some reason if I put the "inline" keyword in either one of the implementation and declaration, as well as in both places, I get linker errors as shown:

 Creating library C:DOCUME~1STANLEYLOCALS~1TEMPMEX_HN~1emplib.x and object C:DOCUME~1STANLEYLOCALS~1TEMPMEX_HN~1emplib.exp 
mexfunction.obj : error LNK2019: unresolved external symbol "public: int __thiscall NeedleUSsim::GetTplLSize(void)" (?GetTplLSize@NeedleUSsim@@QAEHXZ) referenced in function _mexFunction 
mexfunction.mexw32 : fatal error LNK1120: 1 unresolved externals 

  C:PROGRA~1MATLABR2008BBINMEX.PL: Error: Link of 'mexfunction.mexw32' failed. 

What needs to be in order to get rid of this error (i.e. what am I doing wrong in terms of making these inline member functions)?

Answer 1

You need to put function definition into the header then. The simplest way to hint the compiler to inline is to include method body in the class declaration like:

class NeedleUSsim
{
  // ...
  int GetTplLSize() const { return sampleDim[1]; }
  // ...
};

or, if you insist on separate declaration and definition:

class NeedleUSsim
{
  // ...
  int GetTplLSize() const;
  // ...
};

inline int NeedleUSsim::GetTplLSize() const
{ return sampleDim[1]; }

The definition has to be visible in each translation unit that uses that method.