<type_traits> lets templates ask questions about types at compile time; if constexpr (C++17) keeps only the branch that applies, so each instantiation compiles just the code it needs.

Asking questions about types

C++

#include <iostream>
#include <string>
#include <type_traits>

int main() {
    std::cout << std::boolalpha
              << std::is_integral_v<int> << ' '
              << std::is_floating_point_v<int> << ' '
              << std::is_pointer_v<char*> << ' '
              << std::is_same_v<std::remove_const_t<const int>, int> << ' '
              << std::is_trivially_copyable_v<std::string> << ' '
              << std::is_base_of_v<std::exception, std::runtime_error> << '\n';
    static_assert(std::is_same_v<std::decay_t<const int&>, int>);
    static_assert(sizeof(long long) == 8, "this code assumes 64-bit long long");
}

Output

true false true true false true

Trait	Question / transformation
`is_integral_v<T>`, `is_floating_point_v<T>`, `is_arithmetic_v<T>`	Category checks
`is_same_v<A, B>`	Exactly the same type?
`is_base_of_v<B, D>`, `is_convertible_v<From, To>`	Relationships
`remove_reference_t<T>`, `remove_const_t<T>`, `decay_t<T>`	Strip qualifiers (decay = what pass-by-value does)
`conditional_t<Cond, A, B>`	Compile-time ternary on types
`enable_if_t<Cond, T>`	Remove an overload unless `Cond` (pre-C++20 SFINAE)

`if constexpr`

C++

#include <iostream>
#include <string>
#include <type_traits>
#include <vector>

template <typename T>
std::string describe(const T& value) {
    if constexpr (std::is_integral_v<T>)
        return "integer " + std::to_string(value * 2);     // only compiled for integers
    else if constexpr (std::is_floating_point_v<T>)
        return "float " + std::to_string(value);
    else if constexpr (requires { value.size(); })         // C++20 requires-expression
        return "sized thing of " + std::to_string(value.size());
    else
        return "something else";
}

int main() {
    std::cout << describe(21) << '\n' << describe(0.5) << '\n'
              << describe(std::vector<int>{1, 2, 3}) << '\n' << describe(nullptr) << '\n';
}

Output

integer 42
float 0.500000
sized thing of 3
something else

Writing your own trait

C++

#include <iostream>
#include <list>
#include <type_traits>
#include <vector>

template <typename T> struct is_vector : std::false_type {};
template <typename T, typename A> struct is_vector<std::vector<T, A>> : std::true_type {};
template <typename T> inline constexpr bool is_vector_v = is_vector<T>::value;

int main() {
    std::cout << is_vector_v<std::vector<int>> << is_vector_v<std::list<int>> << is_vector_v<int> << '\n';
}

Output

What is SFINAE?