If your function must not throw, declare it `noexcept`

Reason

If an exception is not supposed to be thrown, the program cannot be assumed to cope with the error and should be terminated as soon as possible. Declaring a function noexcept helps optimizers by reducing the number of alternative execution paths. It also speeds up the exit after failure.

Example

Put noexcept on every function written completely in C or in any other language without exceptions. The C++ Standard Library does that implicitly for all functions in the C Standard Library.

Note

constexpr functions can throw when evaluated at run time, so you might need conditional noexcept for some of those.

Example

You can use noexcept even on functions that can throw:

vector<string> collect(istream& is) noexcept
{
    vector<string> res;
    for (string s; is >> s;)
        res.push_back(s);
    return res;
}

If collect() runs out of memory, the program crashes. Unless the program is crafted to survive memory exhaustion, that might be just the right thing to do; terminate() might generate suitable error log information (but after memory runs out it is hard to do anything clever).

Note

You must be aware of the execution environment that your code is running when deciding whether to tag a function noexcept, especially because of the issue of throwing and allocation. Code that is intended to be perfectly general (like the standard library and other utility code of that sort) needs to support environments where a bad_alloc exception could be handled meaningfully. However, most programs and execution environments cannot meaningfully handle a failure to allocate, and aborting the program is the cleanest and simplest response to an allocation failure in those cases. If you know that your application code cannot respond to an allocation failure, it could be appropriate to add noexcept even on functions that allocate.

Put another way: In most programs, most functions can throw (e.g., because they use new, call functions that do, or use library functions that report failure by throwing), so don't just sprinkle noexcept all over the place without considering whether the possible exceptions can be handled.

noexcept is most useful (and most clearly correct) for frequently used, low-level functions.

Note

Destructors, swap functions, move operations, and default constructors should never throw. See also C.44.

Note

Care must be taken on base virtual functions and functions part of a public interface because declaring a function noexcept is establishing a guarantee that all current and future implementations must abide by. For virtual function, all overriders must also be noexcept and removing noexcept from a function could break calling functions.

Enforcement

• (hard) Flag low-level functions that are not noexcept, yet cannot throw.
• Flag throwing swap, move, destructors, and default constructors.