Constructor not returning usable object

Question

I have a problem with the constructor, which is not working as I'd expect.

If I try to initialize my class like that, it will work and I get a usable object:

vector<float> v;
MyClass<2> a(v);

However, if I try to build a class like below (which should be equivalent) the results are quite unexpected. There is no error message/warning when compiling or running the program. But if you try to use this variable a somewhere and call its methods (for example a.doSomething()), it will crash.

I put some code inside the constructor to notify me if it is used. It turned out that no code inside the constructor was actually executed in this case.

MyClass<2> a(vector<float>());

So I am wondering why this is happening? Is the 2nd declaration illegal?

EDIT: I will post some code of the class

template <int x>
class MyClass {
public:
    vector<float> v;
    MyClass<x>(vector<float> v1) {
      v = v1;
    }

};

The class has a public variable of the same type. So it will just assign the argument to the variable. — dominos, Mar 19 '11 at 17:57
I posted some code of the class. It is not a reference or a pointer, just a variable. — dominos, Mar 19 '11 at 18:04

score 28 · Accepted Answer · edited Jun 20 '20 at 09:12

28

MyClass<2> a(vector<float>());

This is not a variable declaration. It is the declaration of a function named a that returns a MyClass<2> object and takes as an argument a "pointer to a function that takes no arguments and returns a vector<float>." Confusing? Yes. This is what is referred to as the "most vexing parse."

You need extra parentheses:

MyClass<2> a((vector<float>()));
             ^               ^

Or, you can use copy initialization:

MyClass<2> a = MyClass<2>(vector<float>());

Or, since your constructor isn't explicit, you could use:

MyClass<2> a = vector<float>();

(Though, unless you mean for vector<float> objects to be implicitly convertible to MyClass<N> objects, you probably want to make this constructor explicit.)

A good compiler should warn you about this sort of thing. Visual C++ warns:

warning C4930: 'MyClass<x> a(std::vector<_Ty> (__cdecl *)(void))': prototyped function not called (was a variable definition intended?)

Clang warns:

warning: parentheses were disambiguated as a function declarator
MyClass<2> a(vector<float>());
            ^~~~~~~~~~~~~~~~~

edited Jun 20 '20 at 09:12

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answered Mar 19 '11 at 18:07

James McNellis

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why MyClass<2> a((vector()))? Shouldn't this be the same as MyClass<2> a(vector())? – BlackBear Mar 19 '11 at 18:10
1

@BlackBear: With the extra parentheses, it isn't a valid function declaration, so it gets parsed as a variable declaration with a direct initializer. – James McNellis Mar 19 '11 at 18:14
Yes, it really seems that adding extra parentheses makes it work. Thanks a lot! – dominos Mar 19 '11 at 18:17
Clang error diagnostic is much powerful. +1 for posting Clang's error message. :) – Prasoon Saurav Mar 19 '11 at 18:29

Constructor not returning usable object

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