What does it mean to "ODR-use" something?

Solution 1:

It's just an arbitrary definition, used by the standard to specify when you must provide a definition for an entity (as opposed to just a declaration). The standard doesn't say just "used", because this can be interpreted diversely depending on context. And some ODR-use doesn't really correspond to what one would normally associate with "use"; for example, a virtual function is always ODR-used unless it is pure, even if it isn't actually called anywhere in the program.

The full definition is in §3.2, second paragraph, although this contains references to other sections to complete the definition.

With regards to templates, ODR-used is only part of question; the other part is instantiation. In particular, §14.7 covers when a template is instantiated. But the two are related: while the text in §14.7.1 (implicit instantiation) is fairly long, the basic principle is that a template will only be instantiated if it is used, and in this context, used means ODR-used. Thus, a member function of a class template will only be instantiated if it is called, or if it is virtual and the class itself is instantiated. The standard itself counts on this in many places: the std::list<>::sort uses < on the individual elements, but you can instantiate a list over an element type which doesn't support <, as long as you don't call sort on it.

Solution 2:

In plain word, odr-used means something(variable or function) is used in a context where the definition of it must be present.

e.g.,

struct F {
   static const int g_x = 2;
};

int g_x_plus_1 = F::g_x + 1; // in this context, only the value of g_x is needed.
                             // so it's OK without the definition of g_x

vector<int>  vi;
vi.push_back( F::g_x );      // Error, this is odr-used, push_back(const int & t) expect
                             // a const lvalue, so it's definition must be present

Note, the above push_back passed in MSVC 2013, this behavior is not standard compliance, both gcc 4.8.2 and clang 3.8.0 failed, the error message is: undefined reference to F::g_x