What is the Use of C++ noexcept?

<span>noexcept</span> is a keyword introduced in C++11, used to specify that a function will not throw exceptions. It serves as both a specifier and an operator, playing an important role in performance optimization, code safety, and compiler optimization.

1. <span><span>noexcept</span></span> Basic Usage

1. As a Function Specifier

void my_function() noexcept {    // This function promises not to throw exceptions}

Equivalent to:

void my_function() noexcept(true); // Explicitly indicates it will not throw

If written as

void my_function() noexcept(false) {    // May throw exceptions}

2. <span><span>What is the Use of noexcept</span></span>?

1. Performance Optimization: Enables More Efficient Code Paths

Some standard library functions (such as <span>std::vector::resize</span>, <span>std::swap</span>) choose more efficient implementations when determining if the element type has a <span>noexcept</span> move constructor.

<span>std::vector</span>‘s <span>push_back</span> and <span>noexcept</span>

#include <vector>
#include <iostream>
struct MayThrow {    MayThrow(MayThrow&&) { /* May throw exceptions */ }
    MayThrow&& operator=(MayThrow&&) { return *this; }};
struct NoThrow {    NoThrow(NoThrow&&) noexcept { /* Will not throw */        std::cout << "Moving NoThrow\n";    }
    NoThrow&& operator=(NoThrow&&) noexcept { return *this; }};
int main() {    std::vector<NoThrow> v1;    v1.push_back(NoThrow{}); // Can safely use move constructor    std::vector<MayThrow> v2;    v2.push_back(MayThrow{}); // If move constructor may throw, vector may use copy constructor to ensure exception safety}

If your type supports a <span>noexcept move constructor</span>, <span>std::vector</span> can directly move elements when reallocating memory instead of conservatively copying, resulting in better performance.

2. Exception Safety Guarantee

<span>noexcept</span> is a contract: this function will not throw exceptions.

If a function declared as <span>noexcept</span> actually throws an exception, the program will directly call <span>std::terminate()</span>, terminating immediately.

void func() noexcept {    throw std::runtime_error("oops"); // Directly terminates the program!}
int main() {    func(); // Program crashes, no stack unwinding}

This is intentionally designed in some system-level code—to avoid the uncertainty and overhead of handling exceptions on critical paths.

3. Helps Compiler Optimization

When the compiler knows a function will not throw exceptions, it can:

  • Remove exception handling stack unwinding code
  • Reduce the size of the generated binary
  • Increase the likelihood of inlining
  • Optimize register allocation
void fast_func() noexcept {    // The compiler knows this won't throw exceptions, can optimize aggressively    for (int i = 0; i < 1000; ++i) {        // ...    }}

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