const 类型的绑定如何在 c++ 中工作？

Question

why this binding is ok

int main()
{
    double d = 4.56;
    const int &r = d;

    return 0;
}

but this is not

int main()
{
    double d = 4.56;
    int &r = d;

    return 0;
}

can anyone explain me while the first one compile but second one shows error

Answer 1

When you bind to a converted type (the double has to be converted to an int ), you get a prvalue, since the converted int is a temporary with no address in memory. Therefore, binding a regular reference to it doesn't work, because they can only bind to glvalues. const type references can bind to prvalues, so the first one compiles still. Source: https://en.cppreference.com/w/cpp/language/value_category

Answer 2

An int reference cannot be bound to an object of type double , because the types mismatch.

So in order to make the initialization of the reference work at all, a new temporary object of the correct type int must be created. This is possible because there is an implicit conversion sequence from double to int .

The reference should then bind to this temporary, ie to a rvalue expression, but only const lvalue references are allowed to bind to rvalues. Non- const lvalue references are not allowed to do that, making the second program ill-formed.

Note that in the first program, although the reference is bound to a temporary object that would usually be destroyed at the end of the full-expression that it was created in, binding to a reference extends the lifetime of that temporary object to the lifetime of the reference. So using r in the first program is actually ok.

However, access through r will not refer to d , but to that new temporary object that is independent of d , which may be surprising, and therefore I think it is not a good idea to write like that. Use auto& or const auto& to make sure that r will certainly refer to d and that there is never any implicit conversion happening due to a type mismatch. If you want the conversion, just use int instead of a reference to int .