CODEWITHSUNDEEP
c++printfvariadic-functions
ATaylor
ATaylor

Reputation: 2598

Passing a class to sprintf

Good morning.

I've got a class, which is technically just a wrapper about a char-array. It allocates memory for the string upon it's construction and releases it upon it's destruction. That's not the problem.

The problem arises, whenever I want to pass it to any vararg function (for example sprintf) My compiler throws me an error, stating that it can't copy a non-trivial type.

After a little Google search, it told me, that a 'trivial type' is any type, which can be copied with memcpy, doesn't have any constructors, etc.

Now my question: Is there any way to get the compiler to pass the internal char string with only writing the variable?

And example:

Wrapper Var("World");
sprintf(Buf, "Hello %s", Var);

After this call, it's supposed to be 'Hello World'.

It works, if I write (and call) any function, which accesses the internal stream (like c_str(), but that's not exactly the purpose of the class. It's supposed to be a clean API, which can be passed around like a regular char string, just with the memory management functions.

I've tried:

operator char *() {
    return InternalString;
}

but it doesn't seem to be getting called by sprintf.

Thank you very much for your attention.

Upvotes: 1

Views: 602

Answers (3)

Davidbrcz
Davidbrcz

Reputation: 2397

You can't pass non-POD object as arguments for variable argument functions. Try to have a look at Boost Format, or use C++11 to write you own type safe sprintf function

Upvotes: 1

suitianshi
suitianshi

Reputation: 3340

in practice, adding a member function acting like 'c_str()' is the best solution or you provide an approach to cast the type 'wrapper' to char*

Upvotes: 1

Jonathan Potter
Jonathan Potter

Reputation: 37192

Implicit conversion operators won't work with variable arg functions like sprintf(), since the compiler doesn't know what type the object needs to be converted to. So in this case you'll need to use the c_str() function (or similar), or explicitly cast it using static_cast<char*>(Var).

Upvotes: 2

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