How do I create a header-only library?

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 :(
}

Answer 2

Use header guards as Liz suggests and don't forget to put "inline" before your function methods.

ie

#ifndef MY_HEADER_H_
#define MY_HEADER_H_

inline RetType FunctionName( ParamType1 param1, ParamType2 param2 )
{
    // Function body
    return retType;
}

#endif

Answer 3

另外，我认为你需要避免在只有头文件库的代码中使用全局变量或静态变量。

Answer 4

对在一个地方编译的部件使用标头防护。