const变量和const类型变量之间的区别

Question

What is the difference between:

const variable = 10;

and

const int variable = 10;

Does variable, per the standard, get interpreted as an integral type when no type is defined?

Answer 1

const variable = 10 is not valid C++, while const int variable = 10; is.

The only time (that I can think of) that const variable = 10 would be valid is if you had a type named variable and you had a function with an unnamed parameter of that type, taking a default argument:

typedef int variable;
void foo(const variable = 10);

Answer 2

It means that x is implicitly declared an int. This is not allowed in C++, but in C and to maintain compatibility with C headers or pre-ISO C++ code, a lot of contemporary C++compilers still support this as an option.

My GCC 4.4 compiler here groks "const x=3;" when feed -fms-extensions on the command line (the manual says, that it turns on a couple of lamps which are required to understand MFC code)

UPDATE: I've checked it with VS-2005, you can have implicit int if you use

#pragma warning(disable:4430)

Answer 3

const variable = 10;

几乎所有新的现代C ++编译器都不会编译。

Answer 4

With no strict rules (K&R C etc. Edit : ie old C), int is the type by default. It certainly does not mean the variable has no type, and it does not have anything to do with const.