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Hi everyone,

I'm writing a class that inherits from std::vector in order to add
some additional functionality. I'm having a compiler problem, however,
when I have a function that returns this class. I've drilled down and
been able to create a very simple example of the problem. Here's my
code:

<code>
#include <vector>
#include <iostream>

using std::vector;
using std::cout;
using std::endl;

template <typename T>
class B : public std::vector<T>
{
public:
B(){ cout << "In B ctor" << endl; }

B(B<T> &b) { cout << "In B copy ctor" << endl; }

B<T>& operator=(const B<T> &b) { cout << "In B assign." << endl;
return *this; }

~B(){ cout << "In B destructor" << endl; }

};

B<int> test()
{
B<int> returnVal;
return returnVal;
}

int main()
{
B<int> b;
b = test();
return 0;
}
</code>

I get the following error when compiling:
test.cpp: In function `int main()':
test.cpp:31: error: no matching function for call to
`B<int>::B(B<int>)'
test.cpp:14: note: candidates are: B<T>::B(B<T>&) [with T = int]

Does anyone know of a solution?

Some additional information: any of the following code in 'main'
compiles fine:
<code>
B<int> b;
B<int> b2 = b;
return 0;
</code>

or

<code>
B<int> b;
B<int> b2;
b2 = b;
return 0;
</code>

or

<code>
B<int> b;
B<int> b2(b);
return 0;
</code>

Any help would be greatly appreciated.

On 12 Juni, 16:08, saxman <erll...@gmail.com> wrote:
> Hi everyone,
>
> I'm writing a class that inherits from std::vector in order to add
> some additional functionality. I'm having a compiler problem, however,
> when I have a function that returns this class. I've drilled down and
> been able to create a very simple example of the problem. Here's my
> code:
>
> <code>
> #include <vector>
> #include <iostream>
>
> using std::vector;
> using std::cout;
> using std::endl;
>
> template <typename T>
> class B : public std::vector<T>
> {
> public:
> B(){ cout << "In B ctor" << endl; }
>
> B(B<T> &b) { cout << "In B copy ctor" << endl; }
>
> B<T>& operator=(const B<T> &b) { cout << "In B assign." << endl;
> return *this; }
>
> ~B(){ cout << "In B destructor" << endl; }
>
> };
>
> B<int> test()
> {
> B<int> returnVal;
> return returnVal;
>
> }
>
> int main()
> {
> B<int> b;
> b = test();

The problem is that the compiler converts this to a copy-constructor
call like this 'B<int>(test())', however since your copy-constructor
takes a reference as a parameter this can not compile (since a
reference can not bind to the temporary returned by test()). To solve
this simply change the copy-constructor to 'B(const B<T> &b)'. In
general the parameters to copy-constructors should be const.

--
Erik Wikström

On Jun 12, 10:16 am, Erik Wikström <eri...@student.chalmers.se> wrote:
> On 12 Juni, 16:08, saxman <erll...@gmail.com> wrote:
>
>
>
> > Hi everyone,
>
> > I'm writing a class that inherits from std::vector in order to add
> > some additional functionality. I'm having a compiler problem, however,
> > when I have a function that returns this class. I've drilled down and
> > been able to create a very simple example of the problem. Here's my
> > code:
>
> > <code>
> > #include <vector>
> > #include <iostream>
>
> > using std::vector;
> > using std::cout;
> > using std::endl;
>
> > template <typename T>
> > class B : public std::vector<T>
> > {
> > public:
> > B(){ cout << "In B ctor" << endl; }
>
> > B(B<T> &b) { cout << "In B copy ctor" << endl; }
>
> > B<T>& operator=(const B<T> &b) { cout << "In B assign." << endl;
> > return *this; }
>
> > ~B(){ cout << "In B destructor" << endl; }
>
> > };
>
> > B<int> test()
> > {
> > B<int> returnVal;
> > return returnVal;
>
> > }
>
> > int main()
> > {
> > B<int> b;
> > b = test();
>
> The problem is that the compiler converts this to a copy-constructor
> call like this 'B<int>(test())', however since your copy-constructor
> takes a reference as a parameter this can not compile (since a
> reference can not bind to the temporary returned by test()). To solve
> this simply change the copy-constructor to 'B(const B<T> &b)'. In
> general the parameters to copy-constructors should be const.
>
> --
> Erik Wikström

Thanks for the help and the explanation, that worked perfectly