I have the practice of writing functions that do not have to be in a class in namespaces, so I would like to know if can separate them in source and headers files:

utilities.hpp:

namespace nms {
    static void process();
};

utilities.cpp

void nms::process(){/*...*/}

But like this I only get an error: main.cpp:(.text 0x5): undefined reference to 'nms::process()'. So i would like to know if this is possible in anyway.

CodePudding user response：

In the header file utilities.hpp:

namespace nms {
    static void process();
};

static means the function has internal linkage, meaning it declares a unique function for each translation unit in which the header is included.

The only translation unit (TU) for which the corresponding unique (internal linkage) process() function has a definition is, however, in the TU associated with utilities.cpp, whereas for any other source file which includes utilities.hpp, no definition exists for the TU-local process() function.

This explains why you get undefined reference errors in locations other than utilities.cpp, as soon as that use site requires a definition for the TU-local function. Remove static and the publically-intended process() function will not have internal linkage.

... but NON-class functions?

The static keyword is unfortunately quite overloaded in meaning in C , and static for class member function does not mean the same thing as for a namespace scope function as process() above. For class member function, using the static keyword makes the static members ([class.static].