No viable conversion from 'Class *' to 'Class' C++

Question

I feel like this should be simple but for whatever reason I can't get it to work.

I'm trying to create an instance of a class that can be passed into and edited directly by other functions.

For example:

main()
{
    ClassFoo foo = new ClassFoo();
    someFunction(foo);
}

void someFunction(ClassFoo& f)
{
    f.add("bar");
}

The problem is, when compiling, I end up with this error.

no viable conversion from 'ClassFoo *' to 'ClassFoo'
    ClassFoo foo = new ClassFoo();
             ^    ~~~~~~~~~~~~~~~

It also says that other candidate constructors are not viable, however in my ClassFoo class I do have a constructor that looks like this:

ClassFoo::ClassFoo()
{}

So how would I be able to accomplish editing the ClassFoo variable in functions?

score 14 · Accepted Answer · edited May 23 '17 at 11:51

14

C++ is not Java (or C# I suppose). You should never use the new keyword unless you know that you need to. And it returns a pointer to the newly created class, hence the error you get. Likely, the following would be sufficient:

Class foo;
someFunction(foo);

For a default constructed object, you shouldn't include the () if you're not using new, as this does something completely different (see the most vexing parse)

edited May 23 '17 at 11:51

Community

1
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answered Mar 19 '15 at 02:43

aruisdante

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The only time when you probably need to use `new` in modern C++ is when specifying custom deleters for smart pointers. +1 – vsoftco Mar 19 '15 at 02:45
Thank you for this. Any idea why when exiting the program (just returning 0 in main), it says that `pointer being freed was not allocated`? – Alex Mar 19 '15 at 03:08
@Alex That implies somewhere you have a `delete` call that's trying to delete something that was not allocated with `new`. See: http://stackoverflow.com/questions/22824802/malloc-error-for-object-pointer-being-freed-was-not-allocated-set-a-br . But like how `new` shouldn't show up unless you really know you need it, neither should `delete`. – aruisdante Mar 19 '15 at 03:15
`Class foo();` would be a function declaration. `Class foo{};` would more closely match what you are saying (although if the class were an aggregate then the `{}` would be significant) – M.M Mar 19 '15 at 03:20

score 6 · Answer 2 · answered Mar 19 '15 at 02:46

You're probably used to C# or similar sintax..

However, in C++, there are big differences between these lines:

ClassFoo foo; //local variable at stack
ClassFoo *foo = new ClassFoo(); //local pointer to some memory at heap which is structured like ClassFoo

You probably wanted first line just to create local object. There are numerous tutorials that describe difference between heap and stack.. so check them out

No viable conversion from 'Class *' to 'Class' C++

2 Answers2