Why was the ampersand chosen as the symbol for references in C++?

Question

Does anyone have an idea why the ampersand was chosen as the way to denote references in C++?

AFAIK (though I don't have the book near me), Stroustroup didn't explain that choice, which I find a little odd because the same symbol was already used for address-of in C.

Answer 1

Stroustrup was always very reluctant to introduce a new reserved symbol or name, so he probably used it to avoid making the feature look weird to users of C.

Answer 2

In addition to Earwicker's response which I generally agree with. I would also speculate that since & is the "address-of" operator, it is somewhat fitting. Since a reference in many ways is like passing by address instead of by value.

In addition to that, taking the address of a variable is often referred to as "referencing"

(Yes I know that references don't have to be implemented using pointers under the hood, I am referring to the way they conceptually work).

This is just speculation though.

Answer 3

Who knows why Stroustrup does anything, but my guess is that because the implementation of reference parameters involves passing the address of an lvalue, Stroustrup chose the C address-of operator because it would give C programmers the right idea about the cost model.

Answer 4

Here is my theory on that. I think it has much to do with what operators are valid (syntactically) for symbols. Consider

int a[1]; // a[1] is valid (syntactically)
int *a; // *a is valid
int a(char, bool); // a(<a char>, <a bool>) is valid (function call)
int C::*a; // <a C>.*a is valid

Conceptually, in those declarations what is named with a type (C, char, bool) is substituted with an expression of that type later on. Of course the intention is to reuse as much of the existing language as possible. So i think he used &:

int &a; // &a is valid

The important one is that & is only valid on the kind of expression a reference denotes: For lvalues. References are lvalues (named variables are too) and only for them & can be applied:

int &g(); // &g() is valid (taking the address of the referred to integer)
int g(); // &g() is *not* valid (can't apply to temporary int)

Answer 5

My thought was that there are 2 symbols used in pointers: * and &. since int* means a pointer to an int, probably Stroustrup didn't want to introduce a whole new symbol. Since references are sort of like pointers, he stuck with &. Plus, the only previously valid use of & was to take the address of something, so it was OK to use in declarations.