const关键字在运算符定义中起什么作用？

Question

I don't understand what the const keyword is used for in front of the return type and after the parameter list of this operator definition. This is taken from an example from a book.

const char& operator [] (int num) const
{
    if (num < getlength())
        return Buffer[num];
}

Answer 1

The C++ const keyword basically means "something cannot change, or cannot delegate operations that change onto other entities." This refers to a specific variable: either an arbitrary variable declaration, or implicitly to this in a member function.

The const before the function name is part of the return type:

const char&

This is a reference to a const char , meaning it is not possible to assign a new value to it:

foo[2] = 'q'; // error

The const at the end of the function definition means "this function cannot change this object and cannot call non-const functions on any object." In other words, invoking this function cannot change any state.

const char& operator [] (int num) const {
  this->modifySomething(); // error
  Buffer.modifySomething(); // error
  return Buffer[num];
}

The goal of const -correctness is a big topic, but the short version is being able to guarantee that immutable state actually is immutable. This helps with thread safety and helps the compiler optimize your code.

Answer 2

这意味着您正在调用此方法的实际对象不会更改，并且如果尝试更改它，则会出现编译器错误。