std::vector resize() 仅在 clear() 后有效

Question

I have a vector object:

std::vector<std::vector<MyClass>> _matrix;

It is 2d array with some data. When i trying to resize the dimensions with:

_matrix.resize(_rows, std::vector<MyReal>(_colms)); //_rows and _colms are ints

this command simply does nothing to the object. So to resize it i have to call first to:

_matrix.clear();

and then:

_matrix.resize(_rows, std::vector<MyReal>(_colms)); 

Of course, I'm losing the data. (In my case it doesn't matter)

Is this expected behaviour?

Answer 1

From the docs for vector::resize :

_Val: The value of new elements added to the vector if the new size is larger that the original size.

Only the new rows get vectors with additional columns ( std::vector<MyReal>(_colms) ). resize will not change the existing rows.

Update: To resize the entire vector properly, iterate over the existing rows and resize those vectors, then add the new rows. Something like this should work:

for (size_t i = 0; i < _matrix.size(); i++)
  _matrix[i].resize(_colms);
_matrix.resize(_rows, std::vector<MyReal>(_colms));