C++ Invalid operands to binary

Question

I'm a beginner to C++ and I'm having a problem with this code, which is supposed to display the scores during the Superbowl final:

#include <iostream>

enum POINTS { EXTRA_POINT = 1, SAFETY = 2, FIELD_GOAL = 3, TOUCHDOWN =6 };

unsigned short giantsScore = 0, patriotsScore = 0;

int main()
{
std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n";

std::cout << " Giants: " << giantsScore = giantsScore + SAFETY << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n"; 

std::cout << " Giants: " << giantsScore = giantsScore + TOUCHDOWN + EXTRA_POINT << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n";

std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore = patriotsScore + FIELD_GOAL << "\n\n";

std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore = patriotsScore + TOUCHDOWN + EXTRA_POINT << "\n\n";

std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore = patriotsScore + TOUCHDOWN + EXTRA_POINT  << "\n\n";

std::cout << " Giants: " << giantsScore = giantsScore + FIELD_GOAL << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n";

std::cout << " Giants: " <<  giantsScore = giantsScore + FIELD_GOAL << "\n";
std::cout << " Patriots: " << patriotsScore << "\n\n";

std::cout << " Giants: " << giantsScore << "\n";
std::cout << " Patriots: " << patriotsScore = patriotsScore + FIELD_GOAL + EXTRA_POINT  << "\n\n";

return 0;
}

Ignoring that this is quite inelegant, when I run this through the compiler, G++, I get the error message

error: invalid operands of types 'int' and 'const char [2]' to binary 'operator<<'

If I remove the constants and add them in before each std::cout, then it runs fine. I just wanted to know why I can't add the constants during each output line?

Answer 1

Your error message states: int << char, which of course is an odd operation.

It is because of operator priorities.

Each operator has a priority meaning it will evaluate before or after the other operators are evaluated.

+ evaluates before =

and << should be evaluated after = had cout<<"stuff" been its original purpose.

<< is originally the bit-shift operator (still is), so that is why you are experiencing this odd behaviour. Add parentheses and you'll be good.

Answer 2

Check http://cs.smu.ca/~porter/csc/ref/cpp_operators.html for an overview of the priority rules of operators. When you write this:

std::cout << " Patriots: " << patriotsScore = patriotsScore + FIELD_GOAL + EXTRA_POINT  << "\n\n";

Then according to the priority rules, the + operator will be executed first, giving you this:

std::cout << " Patriots: " << patriotsScore = result  << "\n\n";

Then the << operator is executed, which means also `result << "\n\n". But this operator is not defined between int and char[2].

To solve your problem, put parenthesis around the assignment operation, like this:

std::cout << " Patriots: " << (patriotsScore = patriotsScore + FIELD_GOAL + EXTRA_POINT)  << "\n\n";