about c++ conversion : no known conversion for argument 1 from ‘[some_class]' to ‘[some_class]&’

Question

I'm working on C++, and had an error that I didn't know the exact reason. I've found the solution, but still want to know why.

    class Base
    {
        public:
               void something(Base& b){}
    };

    int main()
    {
        Base b;
        b.something(Base());
        return 0;           
    }

when I compile the code, I got this following error :

abc.cpp:12:20: error: no matching function for call to ‘Base::something(Base)’
abc.cpp:12:20: note: candidate is:
abc.cpp:6:7: note: void Base::something(Base&)
abc.cpp:6:7: note:   no known conversion for argument 1 from ‘Base’ to ‘Base&’

but when I replaced b.something(Base()) into

Base c;
b.something(c);

the error is gone, I'm wondering why??? aren't they have the same type? It only matters how I write it, but the meaning should be the same???

Thanks Guys!

Seems like a valid and useful question - upvoted to counter the unexplained downvote. — CompuChip, Nov 27 '13 at 16:22
@CompuChip people like you make this community a lot better. — Anoroah, Jul 14 '17 at 11:44

score 45 · Accepted Answer · edited Jul 10 '17 at 19:56

45

You are passing a temporary Base object here:

b.something(Base());

but you try to bind that to a non-const lvalue reference here:

void something(Base& b){}

This is not allowed in standard C++. You need a const reference.

void something(const Base& b){}

When you do this:

Base c;
b.something(c);

you are not passing a temporary. The non-const reference binds to c.

edited Jul 10 '17 at 19:56

urelement

103
2

answered Nov 27 '13 at 16:16

juanchopanza

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+1 good point on the fact this is allowed for const references – Ivaylo Strandjev Nov 27 '13 at 16:18
1

Maybe also state that with vc this might compile (we noticed this as we migrated to g++ :D ) – Sebastian Hoffmann Nov 27 '13 at 16:21
@Paranaix VS doesn't allow it either if you have the right compiler flags set (`/W4` and `/WX`) – Praetorian Nov 27 '13 at 16:43
Thats why I wrote 'might'. As it is an extension you can of course deactivate it. Although this breaks the windows headers AFAIK as they arent standart conform either. – Sebastian Hoffmann Nov 27 '13 at 16:43
1

This answer should be updated to catch up with c++11, the solution can be add a function with r-value reference. – prehistoricpenguin Sep 26 '17 at 04:48

score 2 · Answer 2 · answered Nov 27 '13 at 16:17

2

In the first case you attempt to pass a (non-const)reference to a temporary as argument to a function which is not possible. In the second case you pass a reference to an existing object which is perfectly valid.

answered Nov 27 '13 at 16:17

Ivaylo Strandjev

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about c++ conversion : no known conversion for argument 1 from ‘[some_class]' to ‘[some_class]&’

2 Answers2

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