一、认识std::true_type和std::false_type
std::true_type和std::false_type实际上是类型别名,源码如下:
template <class _Ty,
_Ty _Val>
struct integral_constant {
// convenient template for integral constant types
static constexpr _Ty value = _Val;
using value_type = _Ty;
using type = integral_constant;
constexpr operator