1. Problem It Solves
C++20 standardizes several low-level and numeric utilities that replace unsafe casts, hand-written bit tricks, magic constants, and repetitive wrappers. It makes an important assumption visible and checkable.
2. Prerequisites
Object representation, integers, functions, and numeric operations.
You should be able to compile a short program and read its output.
3. Core Idea
Each utility names one narrow intent: copy representation, identify byte order, inspect bits, use a constant, find a safe middle, or pre-bind leading arguments. Read std::bit_cast as a precise promise; runtime preconditions still belong to the programmer.
4. Minimal Syntax
auto bits = std::bit_cast<std::uint32_t>(value);5. How It Works
The program introduces the smallest relevant form of
std::bit_cast.It applies the feature to fixed data while required owners remain in scope.
It prints one result that can be checked against the source.
6. Common Mistakes
bit_castrequires equal size and trivially copyable types; endianness still matters when bytes cross a file or network boundary.Also check the required header, C++20 library support, lifetime, and deduced types.
7. When to Use It
Use it when the exact standardized operation matches a low-level or numeric task.
Avoid it when a semantic value conversion is needed instead of representation copying.
8. Simple Example
The program inspects the bits of 1.0f, reports native endian, counts bits, uses pi, computes a midpoint, and binds an addend. The companion .cpp uses no input, so its result is easy to reproduce.
Complete sample code
Source file
cpp20/36_bit_endian_math_midpoint_bind_front/main.cpp
// Day 36: bit_cast, Endian, Bit Operations, Math Constants, midpoint, and bind_front
#include <bit>
#include <cstdint>
#include <functional>
#include <iostream>
#include <numeric>
#include <numbers>
int add(int a, int b) {
return a + b;
}
int main() {
float value = 1.0F;
auto bits = std::bit_cast<std::uint32_t>(value);
auto add_ten = std::bind_front(add, 10);
std::cout << "bits = " << bits << '\n';
std::cout << "popcount = " << std::popcount(0b101101u) << '\n';
std::cout << "little endian = "
<< std::boolalpha << (std::endian::native == std::endian::little) << '\n';
std::cout << "pi = " << std::numbers::pi << '\n';
std::cout << "midpoint = " << std::midpoint(2, 10) << '\n';
std::cout << "bound add = " << add_ten(5) << '\n';
}
9. Key Takeaways
std::bit_castexpresses the central C++20 idea of this day.The example isolates one behavior with fixed data.
Compiler checks do not replace lifetime and runtime reasoning.
Prefer the smallest interface that states the real requirement.
10. Self-Check Questions
Easy — What is the main job of
std::bit_castin the minimal example?Medium — Why is
std::midpoint(a,b)safer than(a+b)/2for large integers?Hard — Why does a successful
bit_castnot make the resulting integer a portable serialized representation across different endian machines?