DIfference between macros and templates - compilation/build time

Question

Macros are expanded by preprocessors and templates are by compilers.

But, in terms of compilation/build time, which one takes longer?

Answer 1

Templates undoubtedly take longer.

However, templates are significantly more powerful and obey C++ syntactical rules, whereas macros do not.

The reason Templates take longer is because you can have a template which is recursive, and all of those recurrences need to be generated. This is the foundation upon which looping constructs in Template Metaprogramming are built. Macros, in contrast, cannot call themselves, and are therefore limited to a single expansion.

For example, take the following code, shamelessly stolen from Wikipedia:

template <int N>
struct Factorial 
{
    enum { value = N * Factorial<N - 1>::value };
};

template <>
struct Factorial<0> 
{
    enum { value = 1 };
};

// Factorial<4>::value == 24
// Factorial<0>::value == 1
void foo()
{
    int x = Factorial<4>::value; // == 24
    int y = Factorial<0>::value; // == 1
}

Note how the factorial is calculated at compile time, and for the first call (Factorial<4>), the compiler needs to expand the template 5 times. Macros cannot do this.