std::vector::reserve() vs. std::vector::resize(): When Should I Use Which?

Understanding the Difference: std::vector::resize() vs. std::vector::reserve()

The topic of std::vector::reserve() vs. std::vector::resize() has sparked discussion within the programming community. This article aims to clarify their distinct roles in vector manipulation.

std::vector::reserve()

std::vector::reserve() allocates memory for the specified number of elements but does not resize the vector itself. The vector maintains its original logical size. This method is beneficial when you anticipate adding elements to the vector and want to optimize performance by preallocating memory.

std::vector::resize()

std::vector::resize() modifies the vector's size to the specified number of elements. If the vector needs to be expanded, it will allocate additional memory. Unlike reserve(), resize() initializes any new elements to their default values.

Example Usage

Consider the code provided in the question:

void MyClass::my_method()
{
    my_member.reserve(n_dim);
    for (int k = 0; k < n_dim; k++)
        my_member[k] = k;
}

According to the response, using reserve() here is incorrect. To correctly write elements to the vector, one should use resize():

void MyClass::my_method()
{
    my_member.resize(n_dim);
    for (int k = 0; k < n_dim; k++)
        my_member[k] = k;
}

Visual Studio 2010 SP1 Behavior

The mentioned "crashing" behavior of the code in Visual Studio 2010 SP1 is consistent with the correct behavior described in the response. Attempting to access an element beyond the vector's logical size (in this case, my_member[5]) before resizing the vector results in undefined behavior.

Reserve and Invalidation

It's important to note that using reserve() and then adding elements through push_back() may invalidate references, iterators, or pointers to data in the vector. This is because reserve() allocates memory without resizing, and when new elements are added, they may be placed in a different location.

In conclusion, std::vector::reserve() and std::vector::resize() have distinct purposes. Reserve() preallocates memory, while resize() modifies the vector's size and initializes new elements. Understanding their differences is essential for efficient vector manipulation.

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