C++ Templates: implicit conversion, no matching function for call to ctor

Question

template<class T>
class test
{
    public:
        test()
        {
        }

        test(T& e)
        {
        }

};

int main()
{

    test<double> d(4.3);

    return 0;
}

Compiled using g++ 4.4.1 with the following errors:

g++ test.cpp -Wall -o test.exe
test.cpp: In function 'int main()':
test.cpp:18: error: no matching function for call to 'test<double>::test(double)
'
test.cpp:9: note: candidates are: test<T>::test(T&) [with T = double]
test.cpp:5: note:                 test<T>::test() [with T = double]
test.cpp:3: note:                 test<double>::test(const test<double>&)
make: *** [test.exe] Error 1

However, this works:

double a=1.1;
test<double> d(a);

Why is this happing? Is it possible that g++ cannot implicitly convert literal expression 1.1 to double? Thanks.

Answer 1

It's due to the reference (&) in your constructor definition. You can't pass a constant value by reference like that, only a variable like in your second example.

Answer 2

You cannot bind a double literal to a (non-const) double&.

Did you mean to pass it as a T const& or by value instead? (Either works for the code you've given so far.)

Answer 3

You can't take a non-const reference to a temporary. Try changing your constructor to

    test(const T& e)
    {
    }

or pass by value:

    test(T e)
    {
    }

Answer 4

You're passing the double 1.1 to a non-const reference T&. This means you'd have to pass a valid lvalue to the constructor such as by doing:

double x = 4.3;
test<double> d(x);

Make the constructor take a const reference (const T&) and it works, because you are allowed to bind temporaries (rvalues) to const references, and 4.3 is technically a temporary double.