How to get a pointer to a 2D std::vector's contents

Given

std::vector<char> theVec

I can get the pointer as follows

char* cc = &theVec[0]

What if the vector is declared as follows:

std::vector<std::vector<char> > theVec

How do you get a pointer to the head of theVec into a char*

开发者_运维问答char cc* = 

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How do you get a pointer to the head of theVec [as a char*]?

You can't. The head of your vector is of type std::vector<char>, not char, and a std::vector<std::vector<char> > doesn't store its data in one contiguous block.

Instead, you can do this:

std::vector<char> theVec;
theVec.resize(xSize*ySize);
char cc* = &theVec[0];
char tmp = theVec[x*xSize + y];//instead of theVec[x][y]

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std::vector<char> * p = &theVec[0];

If you want a pointer to the first char, then:

char * p = &theVec[0][0];

However, be careful, because iterating that pointer past the first sub-vector will not move it into the next vector like would happen with a multidimensional array. To get a pointer to the head of the next sub-vector, you would use:

p = &theVec[1][0];