Question

I’d like to package a library I’m working on as a header-only library to make it easier for clients to use. (It’s small and there’s really no reason to put it into a separate translation unit) However, I cannot simply put my code in headers because this violates C++’s one definition rule. (Assuming that the library header is included in multiple translation units of a client project)

How does one modify a library to make it header-only?

Answer 1

You can use the inline keyword:

// header.hpp (included into multiple translation units)

void foo_bad() {} // multiple definitions, one in every translation unit :(

inline void foo_good() {} // ok :)

inline allows the linker to simply pick one definition and discard the rest.

(As such, if those definitions don’t actually match, you get a good dose of undefined behavior…!)

---

As an aside, member functions defined within a class-type, are implicitly marked inline:

struct myclass
{
    void i_am_inline_implicitly()
    {
        // because my definition is here
    }

    void but_i_am_not();
    void neither_am_i();
};

inline void myclass::but_i_am_not()
{
    // but that doesn't mean my definition cannot be explicitly inline
}

void myclass::neither_am_i()
{
    // but in this case, no inline for me :(
}