Why operator [] is not allowed on std::auto_ptr

Why operator [] is not allowed on std::auto_ptr?开发者_JS百科

#include <iostream>

using namespace std ;

template <typename T>
void foo( T capacity )
{
    auto_ptr<T> temp = new T[capacity];

    for( size_t i=0; i<capacity; ++i )
        temp[i] = i; // Error
}

int main()
{
    foo<int>(5);
    return 0;
}

Compiled on Microsoft Visual C++ 2010.

Error: error C2676: binary '[' : 'std::auto_ptr<_Ty>' does not define this operator or a conversion to a type acceptable to the predefined operator

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The reason is that auto_ptr will free the content using delete instead of delete[], and so auto_ptr is not suitable for handling heap-allocated arrays (constructed with new[]) and is only suitable for handling single, heap-allocated arrays that were constructed with new.

Supporting operator[] would encourage developers to use it for arrays and would mistakenly give the impression that the type can support arrays when, in fact, it cannot.

If you want a smartpointer-like array class, use boost::scoped_array.

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Because std::auto_ptr is not intended to be used with arrays.

Besides, In your sample

std::auto_ptr<T> temp = new T(capacity); // T=int, capacity=5

actually allocates a single int and initializes it with capacity. It does not create an array of integers as you seem to have intended.

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Because auto_ptr is designed to hold a pointer to a single element; it will use delete (specifically not delete[]) on its destruction.

Your example is not doing what (I think) you think it does. Or at least the name capacity is misleading, because you are only allocating a single element (and assigning the value of capacity` to it). Your for loop has no sensible meaning.

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auto_ptr and other smart pointers are only intended to store a pointer to a single object. This is because they use delete in the destructor, rather than delete[], which would be needed if it were storing a pointer to an array.

If you need to wrap an array of objects in a smart pointer, the standard library doesn't offer anything to help. However, Boost does offer scoped_array, which behaves similar to std::auto_ptr and is made to hold arrays of objects created by new[].