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what's wrong with this code in VC++ 2008 Ex

#include <conio.h>
#include <iostream>
using namespace std;

class Allocate
{
private:

	void* v;
	unsigned n;
	__int8 dummy_ret;

	void* Alloc(unsigned& n)
	{
		void* ret;
		if(n == 0)
			ret = 0;
		else
		{
			ret = new(nothrow) char[n];
			if(ret == 0)
				n = 0;
		}
		return ret;
	}

	void Free()
	{
		if(n != 0)
			delete [] v;
		n = 0;
	}

public:
	unsigned GetSize()
	{
		return n;
	}

	void SetSize(unsigned size)
	{
		SetData(v,size);
	}

	void* GetData()
	{
		return v;
	}

	void SetData(void* data,unsigned size)
	{
		if(size == 0)
			return;
		char* w = (char*)Alloc(size);
		for(unsigned i = 0; i < size; i++)
			w[i] = ((char*)data)[i];
		Free();
		v = w;
		this -> n = size;
	}

	__int8& operator [](unsigned i)
	{
		if(i < n)
			return ((char*)v)[i];
		else
			return dummy_ret;
	}

	Allocate& operator =(const Allocate& a)
	{
		SetData(a.GetData(),a.GetSize());
		return *this;
	}

	Allocate& operator ++()
	{
		SetSize(GetSize() + 1);
		return *this;
	}

	Allocate(unsigned n)
	{
		v = Alloc(n);
		this -> n = n;
	}

	Allocate(void* w,unsigned size)
	{
		n = 0;
		SetData(w,size);
	}

	Allocate(const Allocate& a)
	{
		*this = a;
	}

	~Allocate()
	{
		dummy_ret = 0;
		Free();
	}
};

int main()
{
	Allocate a("hello",6);
	cout<< a[0];
	_getch();
}

Error 1 error C2662: 'Allocate::GetData' : cannot convert 'this' pointer from 'const Allocate' to 'Allocate &' g:\important files\my documents\borland studio projects\console.cpp 72
Error 2 error C2662: 'Allocate::GetSize' : cannot convert 'this' pointer from 'const Allocate' to 'Allocate &' g:\important files\my documents\borland studio projects\console.cpp 72

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  • Latest Post Latest Post by CppBuilder2006

Dani AI

Generated

and correctly nailed the immediate compiler error: the getters are called on a const reference, so those accessors must be const. Beyond that, the class has several other subtle issues that will bite later — fixing constness is only the first step.

Key practical fixes and gotchas:

  • Avoid self-assignment bugs in operator=. The current implementation copies from a and frees your storage first, which can lead to use-after-free if this == &a. Use a self-assignment check or the copy-and-swap idiom for strong exception safety.
  • Treat a request to resize to zero as a real shrink: free the buffer and set n (the member) to zero. Currently SetSize(0) returns early and leaks.
  • Provide both const and non-const operator[] overloads and make the element type explicit (e.g., char/unsigned char) so streaming (cout << a[0]) behaves predictably.
  • Initialize dummy_ret (it’s returned by reference on out-of-range access). Uninitialized data here is undefined behavior.
  • Prefer size_t for sizes and consider std::vector<char>, std::string, or std::unique_ptr<char[]> to avoid manual new[]/delete[].

A compact copy-and-swap pattern to replace fragile assignment:

void swap(Allocate& other) noexcept {
    std::swap(v, other.v);
    std::swap(n, other.n);
}

Allocate& operator=(Allocate other) {
    swap(other);
    return *this;
}

Also add a const overload for the getters and for operator[]. These changes fix the original compile error and make the class robust, exception-safe, and easier to maintain.

Recommended Answers

All 3 Replies

Member Avatar for Narue

GetData and GetSize are called on objects that are qualified as const. In such a case the compiler has to assume that both of those functions modify the state of the object, so it disallows the call. You can fix it by qualifying the two member functions as const since they don't actually modify the state:

unsigned GetSize() const
{
	return n;
}

void* GetData() const
{
	return v;
}
Member Avatar for JasonHippy

Your GetData and GetSize functions need to be declared const
e.g.

unsigned GetSize() const // This
	{
		return n;
	}
void* GetData() const // and this needs to be const
	{					
		return v;
	}

Because the parameter 'a' to the Allocate classes operator = override function has been declared as a constant reference to an Allocate object, then the GetData and SetSize functions have to be declared as const otherwise the compiler thinks that they might attempt to modify the Allocate object....If you follow me!

Cheers for now,
Jas.

edit:
Dammit, beaten to the punch again!

Edited by JasonHippy because: n/a
Member Avatar for CppBuilder2006

Oops
u proved I'm wrong
thanks

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