Is this `std::vector`?

Is this std::vector? Consider this snippet, whose behavior is not surprising at all:

#define Tc template<class...> class

template<Tc T, Tc U> struct is_same : std::false_type {};
template<Tc T> struct is_same<T, T> : std::true_type {};

namespace A {
    using std::vector;
}
namespace B {
    template<class... Ts> using vector = std::vector<Ts...>;
}

static_assert(is_same<std::vector, std::vector>::value);
static_assert(is_same<A::vector, std::vector>::value);
static_assert(not is_same<B::vector, std::vector>::value);

And this snippet: just because C<T> is vector<T> doesn’t mean C<T> is always vector<T>!

#include <vector>
#include <type_traits>

template<template<class...> class C>
struct Foo {
    static_assert(std::is_same_v<C<int>, std::vector<int>>);
    static_assert(not std::is_same_v<C<float>, std::vector<float>>);
};

For example:

template<class T> using Bar =
    std::conditional_t<std::is_same_v<T, int>, std::vector<int>, void>;

template struct Foo<Bar>;  // OK

Posted 2019-01-17

metaprogramming