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Beware: each user-defined type is a different data type

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• How does C identify a data type
  • Fact:

    The C-compiler uses name equivalence to determine if 2 data types are equal

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  • What does this mean:

    If a variable x (in one file) is defined a typeA and a variable y (possibly in another file) is defined a typeB, then: Variables x and y are of the same data type if and only if the name typeA is equal to the name typeB

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• Example of name equivalence
  • Consider the following program:

    struct typeA { int a; }; struct typeB { int a; }; struct typeC { float a; }; int main(int argc, char *argv[]) { struct typeA x, x1; struct typeB y; struct typeC z; x = x1; // OK, same type ! x = y; // Bad ! DIFFERENT type x = z; // Bad ! DIFFERENT type }

    When you compile the program, you will get error messages for the statements:

    x = y; x = z; type1.c:24: error: incompatible types when assigning to type 'struct typeA' from type 'struct typeB' type1.c:25: error: incompatible types when assigning to type 'struct typeA' from type 'struct typeC'

    Reason:

    x has type: struct typeA y has type: struct typeB, which is a different type than struct typeA (even though their structures are identical) z has type: struct typeC, which is a different type than struct typeA (this is normal because their structures are different)

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• Postscript
  • The other way that a compiler determines whether 2 data types are the same is using:

    Structure equivalence

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  • In structure equivalence:

    Two data types typeA and typeB are equivalent (= same) if their structure are the same.

    In the above example, struct typeA and struct typeB have the same structure