error: invalid operands to binary == (have 'struct demo' and 'struct demo') [duplicate]

Question

This question already has answers here:

Why whole structure can not be compared in C, yet it can be copied? (6 answers)

Closed 8 years ago.

Code 1:

struct demo
{
    int a;
}d[2];

int main()
{
    d[0].a=5;
    d[1]=d[0];
    return 0;
}

This code works fine

Code 2:

struct demo
{
    int a;
}d[2];

int main()
{ 
    d[0].a=5;
    d[1]=d[0];
    if(d[0]==d[1])
    {
        printf("hello");
    }
return 0;
}

This code gives error

error: invalid operands to binary == (have 'struct demo' and 'struct demo')

Why this error is coming in Code 2 ?

Answer 1

You need to compare the members of the struct yourself, like this:

if(d[0].a ==d[1].a)

structs are not valid operands for equality(==), the operands have to be an arithmetic type or a pointer. We can see this from the draft C99 standard section 6.5.9 Equality operators:

One of the following shall hold:

• both operands have arithmetic type
• both operands are pointers to qualified or unqualified versions of compatible types;
• one operand is a pointer to an object or incomplete type and the other is a pointer to a qualified or unqualified version of void; or
• one operand is a pointer and the other is a null pointer constant.

Answer 2

C has no support for struct comparison. You have to compare the struct yourself by comparing all members one by one.

How do you compare structs for equality in C?