Noexcept Move Operations

In C++, the noexcept specifier is used to indicate that a function does not throw exceptions. When applied to move constructors and move assignment operators, it has a special meaning and can enable significant optimizations.

Consider a generic function that takes an object by value:

1template <typename T>
2void Function(T obj) {
3  // ...
4}

When Function is called with an rvalue (such as a temporary), the compiler has a choice: it can either move from the rvalue or copy from it. If the move constructor of T is noexcept, the compiler knows that moving is guaranteed to succeed and will prefer to move. If the move constructor is not noexcept, the compiler will typically choose to copy to avoid the potential for an exception during the move.

Here's an example:

1#include <iostream>
2
3class Resource {
4public:
5  // Default constructor
6  Resource() {}
7
8  // Copy constructor
9  Resource(const Resource& R) {}
10
11  // Copy assignment
12  Resource& operator=(const Resource& R) {
13    return *this;
14  }
15
16  // Move constructor
17  Resource(Resource&& R) noexcept {}
18
19  // Move assignment
20  Resource& operator=(Resource&& R) noexcept {
21    std::cout << "Using Move Assignment";
22    return *this;
23  }
24};
25
26Resource CreateResource() {
27  Resource Temp;
28  return Temp;
29}
30
31int main() {
32  Resource A;
33  A = CreateResource(); 
34}

1Using Move Assignment

In this case, because Resource's move constructor is marked noexcept, the compiler knows it's safe to move from the temporary returned by CreateResource(). If the move constructor were not noexcept, the compiler would likely choose to copy instead, which could be less efficient.

Marking move operations as noexcept is particularly important for types that are used in standard library containers (like std::vector), because the standard library relies on move operations being noexcept to enable certain optimizations.

In general, if your move operations don't throw exceptions (which they usually shouldn't), mark them noexcept to enable these optimizations. But be careful - if you mark a function noexcept and it does throw, the program will terminate immediately.