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c++linkerlinker-errorsheader-files

Linker error depending on where an instance is declared

I have the following header file:

class LogisticActivationFunction
{
public:
    double evaluate(double x) const;
    double evaluate_derivative(double x) const;
};
LogisticActivationFunction logisticActivationFunction;

Then in main.cpp I include it (and do nothing else) and I get an ld error. If I remove the declaration LogisticActivationFunction logisticActivationFunction, I get no ld error.

The really weird part is that, if I add another header file with just:

#include "logistic_activation_function.h"
LogisticActivationFunction logisticActivationFunction;

And include that instead it compiles just fine, even though the way I though headers work it should be completely equivalent to having the declaration in the other header file.

Any ideas what causes this?

Solution

  • Defining a namespace scope variable in a header is a not a good idea. If the header is included into more than one translation unit (which is quite typical thing to do with a header), then there will be more than one definition, which violates the rules of the language.

    In the header, you can declare a variable without defining by using the extern linkage-specifier. Then, you can define the variable in exactly one source file. Alternatively since C++17, you can declare the variable inline in which case no separate definition in a source file is needed.