为什么typedef不能在这里工作？

Question

I know simple definition of typedef :

typedef is a keyword in C to assign alternative names to types.

Following this definition I tried to implement typedef as following :

int main()
{
    typedef long mylong; //as per my knowledge after this statement mylong will be treated as long

    int long b;  // this works fine
    int mylong c; // but this gives error
}

I tried this on gcc. And following is the error

I know this error means I didn't get actual concept of typedef. Can anybody please tell me where I am wrong?

Answer 1

Unlike #define , typedef is a mechanism for introducing a new name for a type, as opposed to a textual substitution.

Recall that types long int , int long , and long are three synonyms that refer to the same C type. When you use typedef , you make another synonym referring to that same type.

When you use it like this

mylong x = 123;

the usage is correct: mylong is used like a name of a type. However, when you try using it in combination with int , like this

int mylong x = 123;

the compiler reports an error because int mylong does not name a valid type. To the compiler it looks the same as if you wrote, say int float x = 5 or struct mystruct int z = ... .

Answer 2

When you omit the type in C , it's assumed int

So typedef long mylong; is the same as typedef long int mylong; .

Making the offending line be something like this:

int long int c;

Hence the error.

The typedef is a new type* (not text substitution for long ). So you don't need to add an int to make a variable of that type. A simple mylong c; will suffice.

_{*Well, it's a bit more involved than that. The new type is in fact the same as a regular long int , and the two are interchangeable. But for sound domain logic, you should treat it as a new seperate type}

Answer 3

According tp paragraph 2 (Constraints) of section 6.7.2 Type specifiers of the C Standard

Each list of type specifiers shall be one of the following multisets...

— void
— char
— signed char
— unsigned char
— short, signed short, short int, or signed short int
— unsigned short, or unsigned short int
— int, signed, or signed int
— unsigned, or unsigned int
— long, signed long, long int, or signed long int
— unsigned long, or unsigned long int
— long long, signed long long, long long int, or
signed long long int
— unsigned long long, or unsigned long long int
— float
— double
— long double
— _Bool
— float _Complex
— double _Complex
— long double _Complex
— atomic type specifier
— struct or union specifier
— enum specifier
— typedef name

As you see the C Standard does not allow to combine typedef name with other type specifiers.

Answer 4

You misunderstand long / short .

typedef long mylong;

You said "typedef is a keyword in C to assign alternative names to types ." long is actually not type - long int is type. C just allows you to leave out int when you're using short , long or long long So your statement is equal to

typedef long int mylong;