Weird compiler decision

Question

I've got the following code:

enum nums {
  a
};

class cls {
public:
  cls( nums );
};

void function()
{
  cls( a );
}

When I try to compile it with gcc, I get the following error:

test.cpp: In function ‘void function()’:
test.cpp:12:10: error: no matching function for call to ‘cls::cls()’
test.cpp:12:10: note: candidates are:
test.cpp:7:3: note: cls::cls(nums)
test.cpp:7:3: note:   candidate expects 1 argument, 0 provided
test.cpp:5:7: note: cls::cls(const cls&)
test.cpp:5:7: note:   candidate expects 1 argument, 0 provided
make: *** [test] Error 1

If I replace the function with this:

void function()
{
  cls name( a );
}

then everything works. It also works if I use a constructor with two arguments. It does not work if I add "explicit" to the constructor.

I get that gcc is somehow parsing this as defining a variable of type "cls" with the name "a", but I am not familiar with such a syntax for defining variables. To my eyes, this is a statement defining an anonymous temporary variable of type cls, passing "a" is the parameter.

Compiled with gcc 4.6.3.

Any insights?

Thanks, Shachar

Answer 1

Another example of the most embarassing parse problem. The line:

cls( a );

declares a local variable named a of type cls, which is (or should be) initialized by calling the default constructor. Which doesn't exist, whence the error message.

If you really want to construct a temporary object which will be destructed immediately after, you can remove the ambiguity by putting the entire expression in parentheses:

(cls( a ));

A definition cannot occur in parentheses; an expression can.