Example:

int i = 5, j = 4, k = 3, l = 2; 

bool b = (i > j);              // right 
int a = (i > j);                 // wrong: implicit conversion from bool to int 
int a = ((i > j) + (k > l)) // wrong: implicit conversion from bool to int

Exception:

Since smart pointers are designed according to the "safe bool idiom" (they behave correctly in boolean context), it is also allowed to use a smart pointer in a boolean context, e.g.

shared_ptr<A> a_sp = someFunctionReturningSharedPtr();
bool my_bool = a_sp;   // right: smart pointers are implicitly of boolean type