Does vector function reserve() allocates space?

[bajajv]

Hi, I have gone thru a tutorial about vectors in cppreference.com and it says that the reserve() function in vector allocates space for vector. But if I allocate some space for vector and try to give some value using the [] operator, it crashes.

vector<int> vect;
vect.reserve(5);
 
cout << "Capacity - " << vect.capacity() << endl; //it gives 5
 
vect[0] = 10; //it crashes, saying out of range value given
 
 

What is the use of vector function() reserve?

[horace1]

reserve() requests a change in vector capacity ready for a change in size later
http://www.cplusplus.com/reference/stl/vector/reserve/

try running

int main()
{
vector<int> vect;
vect.reserve(5);
 
cout << "Capacity - " << vect.capacity() << endl; //it gives 5
cout << "size - " << vect.size() << endl; //it gives 5
vect.resize(5);
cout << "Capacity - " << vect.capacity() << endl; //it gives 5
cout << "size - " << vect.size() << endl; //it gives 5
vect[4] = 10; //it crashes, saying out of range value given
vect.at(4) = 10; //it crashes, saying out of range value given
cout << vect[4] << endl;
 
}

one would use reserve() to allocate capacity so that the vector does not have to be reallocated every time the current capacity is exceeded
I would also recommend that you use the at() method rather than operator[] as it checks for out of range and throws an exception - operator[] does not throw an exception - the program may carry on and crash later or give a memory segmentation error

[weaknessforcats]

When you reserve space in a vector does not add elements. The operator[] ony works for elements that exist and can cuase a crash if the index value is greater than the size of he vector.

This behavior duplicates the behavior of the [] operator used on an array. vector is required to implement an array hence the same behavior in accessing loctions outside the array bounds.

[bajajv]

Thanks for the responses.