Question

I’m stuck on evaluating a struct as a bool in c++

struct foo {
  int bar;
};
foo* x = (foo*)malloc(sizeof(foo));
int y = ( x ) ? 3 : 4;

This is what happens when it is ran:

could not convert 'x' from 'foo' to 'bool'

When searching for bool operators, I can only find ways to overload the comparator bool operator. Is there a straight up evaluate struct as a bool operator?

I also want to keep foo as plain old data if possible.

Answer 1

This can’t be the correct code for that error. The error states that it can’t convert x from foo to bool, but x is not a foo, it is a foo*. And a foo* is convertable to bool¹, so there would be no error for the code you have given.

If you want to be able to use a foo object as a conditional, you need to provide operator bool member function for your foo class:

struct foo {
  int bar;
  operator bool() { return true; }
};
foo x;
int y = x ? 3 : 4;

In this case, any foo will always evaluate to true. This conversion to bool is considered “unsafe” because your object may be converted to bool in some unexpected places. To get around this, in C++03 you can use the safe bool idiom and in C++11 you can mark your operator bool as explicit:

struct foo {
  int bar;
  explicit operator bool() { return true; }
};

The explicit operator bool will still be used automatically in certain places, such as as a condition in if statements and while loops. There are other answers that cover this.

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¹ A prvalue of arithmetic, unscoped enumeration, pointer, or pointer to member type can be converted to a prvalue of type bool. A zero value, null pointer value, or null member pointer value is converted to false; any other value is converted to true.