问题
A C++ compiler that I will not name lets you take the address of a literal, int *p = &42;
Clearly 42 is an r-value and most compilers refuse to do so.
Why would a compiler allow this? What could you do with this other than shoot yourself in the foot?
回答1:
What if you needed a pointer to an integer with the value of 42? :)
C++ references are much like automatically dereferenced pointers. One can create a constant reference to a literal, like this:
const int &x = 42;
It effectively requires the compiler to initialize a pointer with the address of an integer with the value 42, as you might subsequently do this:
const int *y = &x;
Combine that with the fact that compilers need to have logic to distinguish between a value which has not had its address taken, and one which has, so it knows to store it in memory. The first need not have a memory location, as it can be entirely temporary and stored in a register, or it may be eliminated by optimization. Taking the address of the value potentially introduces an alias the compiler can't track and inhibits optimization. So, applying the & operator may force the value, whatever it is, into memory.
So, it's possible you found a bug that combined these two effects.
回答2:
Because 42 is the answer to life, the universe and everything. When asked for its address it is the answer itself.
回答3:
Tongue slightly (nut by no means totally) in cheek:
I'd say that in C++ application code taking the address of an integer whether lvalue or rvalue is almost always a mistake. Even using integers, for doing anything much more than controlling loops or counting is probably a design error, and if you need to pass an integer to a function which might change it, use a reference.
回答4:
Found something related to rvalue references in C++0x -- move semantics http://www.artima.com/cppsource/rvalue.html
回答5:
It effectively requires the compiler to initialize a pointer with the address of an integer with the value 42
Then why, in some compilers, we can't take the address of a literal directly ?
int* ptr = &10;
The reference:
int& ref = 10;
is almost the same thing as a pointer, though...
来源：https://stackoverflow.com/questions/1166378/why-do-some-c-compilers-let-you-take-the-address-of-a-literal