Could I retrieve the datatype from a variable in C?

Question

Has a way to get the datatype in C?

For example:

int foo;

if (foo is int)
{
    // do something
}

or something like:

if (typeof(foo) == typeof(int))
{
    // do something
}

Thanks in advance.

it's an int, you'll know by lookint at the source code - no need for reflection ! — nos, Feb 26 '11 at 15:44
I do not really understand the question. If you define foo as an int, why would you need to get its type at compile time afterwards; introspection is useful in object-oriented programming to implement polymorphism, but I do not see why you would need it in C. — Greg, Feb 26 '11 at 16:10
Such constructions would be useful to implement function overloading. — PintoDoido, Apr 01 '19 at 16:25

score 13 · Accepted Answer · answered Feb 26 '11 at 15:42

13

This is called type introspection or reflection and is not supported by the C language. You would probably have to write your own reflection library, and it would be a significant effort.

answered Feb 26 '11 at 15:42

sourcenouveau

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Thanks, I've already imagine that. Something like #define MY_INT and later set another variable to hold my custom type, right? Thanks. – user464230 Feb 26 '11 at 15:45
1

or you could define a structure, where one field holds a "typeid" and the other field is larger enough to hold any value you might wish to store in the structure. – Jimmy Feb 26 '11 at 15:54

score 4 · Answer 2 · answered Feb 26 '11 at 15:43

4

There is a typeof extension in GCC, but it's not in ANSI C: http://tigcc.ticalc.org/doc/gnuexts.html#SEC69

answered Feb 26 '11 at 15:43

Jeff Ames

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4

And it's not useful for what OP wants. – R.. GitHub STOP HELPING ICE Feb 26 '11 at 16:01

score 2 · Answer 3 · edited May 23 '17 at 11:54

2

The fact that foo is an int is bound to the name foo. It can never change. So how would such a test be meaningful? The only case it could be useful at all is in macros, where foo could expand to different-type variables or expressions. In that case, you could look at some of my past questions related to the topic:

Type-generic programming with macros: tricks to determine type?

Determining presence of prototype with correct return type

edited May 23 '17 at 11:54

Community

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answered Feb 26 '11 at 16:05

R.. GitHub STOP HELPING ICE

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You didn't get the point... anyway... the question was solved by emddudley – user464230 Mar 02 '11 at 05:46

score 1 · Answer 4 · answered Apr 01 '19 at 16:06

1

Since C11, you can do that with _Generic:

if (_Generic(foo, int: 1, default: 0)) // if(typeof(foo)==int)
{
    // do something
}

answered Apr 01 '19 at 16:06

Petr Skocik

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score 0 · Answer 5 · answered Feb 27 '11 at 03:50

The only time you wouldn't know the type is if the type of foo is defined by a typedef -- if that's the case, your example should reflect it. And why do you need to something dependent on the type? There may well be a way to solve your actual problem, but you haven't presented your actual problem.

Could I retrieve the datatype from a variable in C?

5 Answers5

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