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Two question regarding decltype and typeof:
decltype and typeof operators?typeof become obsolete in C++11?
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In the C++ programming language, decltype is a keyword used to query the type of an expression. Introduced in C++11, its primary intended use is in generic programming, where it is often difficult, or even impossible, to express types that depend on template parameters.
Decltype keyword in C++ Decltype stands for declared type of an entity or the type of an expression. It lets you extract the type from the variable so decltype is sort of an operator that evaluates the type of passed expression. SYNTAX : decltype( expression )
decltype is a compile time evaluation (like sizeof ), and so can only use the static type.
The typeof operator returns the type of its argument, which can be an expression or a type. The language feature provides a way to derive the type from an expression. Given an expression e , __typeof__(e) can be used anywhere a type name is needed, for example in a declaration or in a cast.
There is no typeof operator in c++. While it is true that such a functionality has been offered by most compilers for quite some time, it has always been a compiler specific language extension. Therefore comparing the behaviour of the two in general doesn't make sense, since the behaviour of typeof (if it even exists) is extremely platform dependent.
Since we now have a standard way of getting the type of a variable/expression, there is really no reason to rely on non portable extensions, so I would say it's pretty much obsolete.
Another thing to consider is that if the behaviour is of typeof isn't compatible with decltype for a given compiler it is possible that the typeof extension won't get much development to encompass new language features in the future (meaning it might simply not work with e.g. lambdas). I don't know whether or not that is currently the case, but it is a distinct possibility.
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