Forward declaration with std namespace

[Raistlin]

I am writing a software development kit and would like to use as much forward declarations as possible. One of the used classes is std::fstream, but I do not want to include <fstream> in the header files, since I do not want external includes.

However, how to use a forward declaration as the following without using the std notation ? I could use a void* as work-around, but that does not sound like a clean solution.

class SmallClass;
//class std::fstream; is not accepted by the compiler, for obvious reasons
 
class Bigclass {
public:
...
 
private:
SmallClass* smallObject_;
}

class SmallClass;
//class std::fstream; is not accepted by the compiler, for obvious reasons

class Bigclass {
public:
...

private:
SmallClass* smallObject_;
}

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