Why does "most important const" have to be const?

Question

In http://herbsutter.com/2008/01/01/gotw-88-a-candidate-for-the-most-important-const/ it mentions "most important const" where by C++ deliberately specifies that binding a temporary object to a reference to const on the stack lengthens the lifetime of the temporary to the lifetime of the reference itself. I was wondering why c++ only allows the lifetime of the object to be lengthened when the reference is const and not when it isn't? What is the rational behind the feature and why does it have to be const?

Answer 1

Here's an example:

void square(int &x) {   x = x * x; }  int main() {   float f = 3.0f;    square(f);    std::cout << f << '\n'; } 

If temporaries could bind to non-const lvalue references, the above would happily compile, but produce rather surprising results (an output of 3 instead of 9).

Answer 2

Consider the following:

int& x = 5; x = 6; 

What should happen if this was allowed? By contrast, if you did

const int& x = 5; 

there would be no legal way to modify x.