typedefing一个模板类型是否实例化它？

Question

If I use typedef to rename a specialization of a template class (or struct), will that instantiate the template of that type in the assembly codegen? For example:

template<class T> struct Key
{
    float _time = 0.0f;
    T _value = T();
};
typedef Key<glm::vec3> VectorKey;
typedef Key<glm::quat> QuatKey;

Will this create the Key<glm::vec3> and Key<glm::quat> structs in my file so that I now have (essentially)

template<class T> struct Key
{
    float _time = 0.0f;
    T _value = T();
};
struct VectorKey
{
    float _time = 0.0f;
    glm::vec3 _value = glm::vec3();
};
struct QuatKey
{
    float _time = 0.0f;
    glm::quat _value = glm::quat();
};

I'm wondering if it's a bad idea to include a typedef in the header because if this happened, any files that included this header would then have more code included that is unnecessary.

Answer 1

No, typedefs are purely syntactical. Templates are only instantiated when actually used.

As per the C++14 standard (well, the November draft because I can't link directly to the ISO spec) (emphasis mine)

7.1.3 The typedef specifier
[...]
A name declared with the typedef specifier becomes a typedef-name . Within the scope of its declaration, a typedef-name is syntactically equivalent to a keyword and names the type associated with the identifier in the way described in Clause 8. A typedef-name is thus a synonym for another type. A typedef-name does not introduce a new type the way a class declaration (9.1) or enum declaration does .

Whereas the term "instantiating a template" is the same thing as "introducing a new type".

Imagine that during compilation that each new class/type definition encountered adds it to a "list-of-types". Whenever a template is encountered it is not added to the "list-of-types" until instantiated.