The following is a legit, albeit a little obscure, C89 program. It is also a legit C++11 (and above) program.
int main(void) { auto a = 42; return 0; } |
In C89, auto is used as storage specifier and a‘s type is (implicitly) int. C++ and C (beginning with C99) forbid implicit int declarations, but C++11 changes the meaning of the keyword auto from an automatic storage specifier (which is, conveniently, the default in function scope) to automatically deduced type. 42 is an integer literal (and a pretty good answer, from what I understand), so the deduced type is int (also C89’s default). The following table describes how different versions of C and C++ interpret auto a = 42; in a function scope:
| C89 | C99 | C11 | C++03 | C++11 | |
|---|---|---|---|---|---|
| Storage specifier | explicit auto* | explicit auto* | explicit auto* | explicit auto* | implicit auto |
| Type | implicit int | none | none | none | automatically deduced int |
| Result | int with automatic storage duration | code incorrect | code incorrect | code incorrect | int with automatic storage duration |
*also the default
In the end, it means the exact same thing in C++11 and C89, but for completely different reasons.
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