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    <p>const subscript operators of <code>STL</code> containers can return a reference-to-const because they flat out deny calls to it with indexes that do not exist in the container. Behaviour in this case is undefined. Consequently, as a wise design choice, <code>std::map</code> doesn't even provide a const subscript operator overload.</p> <p>QMap tries to be a bit more accommodating, provides a const subscript operator overload as syntactic sugar, runs into the problem with non-existing keys, again tries to be more accomodating, and returns a default-constructed value instead.</p> <p>If you wanted to keep STL's return-by-const-reference convention, you'd need to allocate a static value and return a reference to <em>that</em>. That, however, would be quite at odds with the reentrancy guarantees that <code>QMap</code> provides, so the only option is to return by value. The <code>const</code> there is just sugar coating to prevent some stupid mistakes like <code>constmap["foo"]++</code> from compiling.</p> <p>That said, returning by reference is not always the most efficient way. If you return a fundamental type, or, with more aggressive optimisation, when <code>sizeof(T)&lt;=sizeof(void*)</code>, return-by-value often makes the compiler return the result in a register directly instead of indirectly (address to result in register) or—heaven forbid—on the stack.</p> <p>The other reason (besides premature pessimisation) to prefer pass-by-const-reference, slicing, doesn't apply here, since both <code>std::map</code> and <code>QMap</code> are value-based, and therefore homogeneous. For a heterogeneous container, you'd need to hold pointers, and pointers are fundamental types (except smart ones, of course).</p> <p>That all said, I almost never use the const subscript operator in Qt. Yes, it has nicer syntax than <code>find()</code>+<code>*it</code>, but invariably, you'll end up with <code>count()</code>/<code>contains()</code> calls right in front of the const subscript operator, which means you're doing the binary search <em>twice</em>. And <em>then</em> you won't notice the miniscule differences in return value performance anyway :)</p> <p>For <code>value() const</code>, though, I agree that it should return reference-to-const, defaulting to the reference-to-default-value being passed in as second argument, but I guess the Qt developers felt that was too much magic.</p>
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    1. POAny ideas why QHash and QMap return const T instead of const T&?
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    1. USMarc Mutz - mmutz
    1. USMarc Mutz - mmutz
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    1. POAny ideas why QHash and QMap return const T instead of const T&?
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    1. CO"std::map's const subscript operator..." But `std::map` doesn't have a `const` subscript operator?
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      1. USCB Bailey
    2. COFair enough, edited it out. Minor detail, though :)
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      1. USMarc Mutz - mmutz
    3. CO"they flat out deny calls to it with indexes that do not exist in the container" but you can do this on std::map, that's one way to insert new values. But even if you meant in the general case, how does it follow on that they don't provide a const subscript operator?
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      1. USCiscoIPPhone
 

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