alt Hacker NewsApparently the C++ standard calls this type deduction. But I've always called it type inference.
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Type inference is when you try to infer a type from its usage. ``auto`` does no such thing, it just copies a known type from source to target. Target has no influence over source's type.
Inference is a constraint solving problem: the type of a variable depends on all the ways it is used. In C++, deduction simply sets the type of a variable from its initializing expression.
95%[1] of the time, deduction is enough, but occasionally you really wish you had proper inference.
[1] percentage made up on the spot.
The same general concepts often have different terminology between different languages. For example, which is better, a parent class or a super class? Or a method function or a member function? Initializer or constructor? Long list of these synonyms.
Type deduction is a form of type inference, a very restricted/crude form of type inference that only considers the type of the immediate expression. The term is used in C++ because it predates the use of auto and was the term used to determine how to implicitly instantiate templates. auto uses exactly the same rules (with 1 single exception) as template type deduction, so the name was kept for familiarity. If instead of initializing a variable, you went through the examples on this website and passed the expression into a template function, the type would be deduced in exactly the same way with the exception of initializer lists.
Type inference is usually reserved for more general algorithms that can inspect not only how a variable is initialized, but how the variable used, such as what functions it's passed into, etc...