Re: conversion from '...' to non-scalar type '...' requested
tthunder@gmx.de wrote:
Hi @all,
My small example does not compile... I know, that this (as always) has
reasons, but I want to know WHY?
Because you cannot bound an rvalue to a non-const reference.
BTW:
I only get errors with g++ (4.x), BCB (6.0),...
VS C++ (2005) works perfectly (without warnings etc.)
class classValue
{
public:
classValue() {}
};
class classHolder
{
public: classHolder(classValue &ami) {} // <== I know, but in
this case reference MUST be non-const
};
classHolder getAHolder()
{
return classValue(); // <--- Error: conversion from
'classValue' to non-scalar type 'classHolder' requested
If you could do that, you'd get several more important problems:
void f(int& i)
{
i = 2;
}
int main()
{
int i = 4;
double d = 1.0;
f(i); // ok
f(d); // ok??
f(4); // ouch!
// what should this output?
std::cout << d;
}
For the second call to f(), the double gets converted to a temporary
int (an rvalue), which gets modified in f(). Do you expect 'd' to get
modified also? It won't and that's misleading, dangerous and illegal.
}
Using a little trick, everthing works well...
Yes, you may do that.
class classValue
{
public:
classValue() {}
classValue &self() { return *this; }
};
class classHolder
{
public: classHolder(classValue &ami) {}
};
classHolder getAHolder()
{ return classValue().self();}
But why must I use this nasty indirection?
Because that's how the language is defined. Note that you may also do
classholder getAHolder()
{
classValue v;
return classHolder(v);
}
Jonathan
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