Converting between C++ std::vector and C array without copying

c++ c arrays stl stdvector
D R picture D R · Nov 14, 2009 · Viewed 54.8k times · Source

I would like to be able to convert between std::vector and its underlying C array int* without explicitly copying the data.

Does std::vector provide access to the underlying C array? I am looking for something like this 

vector<int> v (4,100)
int* pv = v.c_array();

EDIT:

Also, is it possible to do the converse, i.e. how would I initialize an std::vector from a C array without copying?

int pv[4] = { 4, 4, 4, 4};
vector<int> v (pv);

Answer

James McNellis picture James McNellis · Nov 14, 2009

You can get a pointer to the first element as follows:

int* pv = &v[0];

This pointer is only valid as long as the vector is not reallocated. Reallocation happens automatically if you insert more elements than will fit in the vector's remaining capacity (that is, if v.size() + NumberOfNewElements > v.capacity(). You can use v.reserve(NewCapacity) to ensure the vector has a capacity of at least NewCapacity.

Also remember that when the vector gets destroyed, the underlying array gets deleted as well.

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