one definition rule (odr)

the one definition rule is as follows:

1. in a file, a function, variable, type, or template in a given scope, must only have one definition
2. in a program, a (non-inlined) function or (non-inlined) variable in a given scope must only have one definition

knowing the odr can save you loads of time when compiling (or linking) larger projects.

for example, the following example violates part 1 of the odr:

int main()
{
    int x = 0;
    int x = 0;
}

when the compiler reaches the second line, it will error out, complaining about a redefinition of x.

on the other hand, the following example violates part 2 of the odr:

// declarations.h
int f();

// a.cpp
int f()
{
    return 1;
}

// b.cpp
int f()
{
    return 1;
}

// main.cpp
#include "declarations.h"

int main()
{
    f();
}

in this case, when compiled with g++ a.cpp b.cpp main.cpp, a.cpp will compile fine, b.cpp will compile fine, but there will be a linker error because of the multiple definitions of f().