What is the purpose of default in C++?

In C++, the default keyword is commonly used in the switch statement as the default label. It signifies that when the value of the expression in the switch statement does not match any case labels, the code block following the default label is executed. This provides a default handling branch for the switch statement to handle unmatched cases.

For example:

int num = 2;
switch (num) {
    case 1:
        // 处理num为1的情况
        break;
    case 2:
        // 处理num为2的情况
        break;
    default:
        // 处理其他情况
        break;
}

In the code above, if the value of num is not 1 or 2, then the code block after the default label will be executed. By default, the default label is optional, but not providing a default handling branch in some cases may lead to logic errors in the program.