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Asked: 2026-05-28T06:45:49+00:00

I thought I’d try beefing up my C++ and OpenGL by looking at the

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I thought I’d try beefing up my C++ and OpenGL by looking at the recently-released Doom 3 source. Much learned so far, but I’ve hit a wall. The class detailed here has methods

float  operator[] (int index) const

and 

float &  operator[] (int index)

whose bodies both read

return ( &x )[ index ];

where x is one of the class’ two data members (the other being y; this class is for 2-vectors).

While I can understand the syntax of each version’s header/prototype, I don’t get why they’re both present.

const seems to appear (or not appear, as preferred) only to distinguish the headers sufficiently to allow compilation. (That is, remove const and VS2010 refuses to compile, similarly if both headers end in const.)

And why return a ref to a float? None of the class’ seven other float-type methods do this, so I’m guessing efficiency isn’t a factor (tho’ maybe this operator’s called vastly more often than the others).

Appreciate any insight as to what’s going on here…

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1 Answer

  1. Editorial Team

    This is a common idiom (known as “const overloading”). See the C++ FAQ: http://www.parashift.com/c++-faq-lite/const-correctness.html#faq-18.12.

    The ambiguity is resolved by whether *this is const or not. On a const object, the const overload is called, in which case it acts in a read-only style. On a non-const object, the non-const is called, in which case it acts in a read/write style.

    Note, crucially, that this is not a way of distinguishing between read and write accesses.

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