Meet the C++ 20 consteval

And a short introduction about constexpr .

Meet the C++ 20 consteval

constexpr is similar to the const specifier, the difference is that it is applicable to more types, eg also static member functions and so on.

Of course, like const and auto, it requires initialization at declaration time. In summary is:

The calculation is done at compile time gaining a much higher performance than if you didn’t use constexpr. Example of use:

constexpr int x = (9 + 3) / ( 2 * 2 ) - 1;

To get runtime data you can use the command: time.

For more information on constexpr see here .

What about consteval ?

consteval can be summarized as an improvement of constexpr . Conceptually speaking, according to cppreference , it is a specifier that declares a function or model to be an immediate function .

This above sounded a bit difficult to understand, but in short it’s that the documentation refers to the order of execution.

constexpr left “doubts” as to what the result would be after multiple uses for functions, programmers had that feeling that it ran at runtime, so consteval was implemented from C++20 to resolve these discrepancies.

Below are some examples of using the consteval

Note that it is used for FUNCTIONS and only compiles with -std=c++20 or higher, let your LSP know! 😃

#include <iostream>

consteval unsigned ret_calc(int first_num){
  return (first_num + 3) / ( 2 * 2 ) - 1;

consteval int make_double(unsigned x){
  return 2 * x;

consteval auto self_mult(auto num){
  return num * num;

consteval static int increment(double x){
  return ++x;

int main(){
  // consteval int y = (9 + 3) / ( 2 * 2 ) - 1; // unlike constexpr, this doesn't compile
  std::cout << ret_calc(9) << '\n';
  std::cout << make_double(3) << '\n';
  std::cout << self_mult(39.6f) << '\n';
  std::cout << increment(6) << '\n';

Useful links:

cpp cppdaily


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Marcos Oliveira

Marcos Oliveira

Software developer

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