cpp使用向量编译错误初始化二维数组

Question

I'd like to initialize 2-D array m_field using initialization list in the constructor. Like in this thread Creating 2-dimensional vector in class C++ , but i get errors listed below.

BoardData.h

#ifndef BOARDDATA_H
#define BOARDDATA_H

#include <vector>

class BoardData
{
    public:
    /** Default constructor */
    BoardData(int rows, int cols);
    /** Default destructor */
    virtual ~BoardData();
    protected:
    private:
    std::vector< std:vector<int> > m_field;

};


#endif // BOARDDATA_H

BoardData.cpp

#include "BoardData.h"

BoardData::BoardData(int rows, int cols) :
                     m_field(rows, std::vector<int>(cols,0))
{
    //ctor
}

BoardData::~BoardData()
{
    //dtor
}

compiler output: (gcc version 4.6.3)

BoardData.h|18|error: template argument 1 is invalid|
BoardData.h|18|error: template argument 2 is invalid|
BoardData.cpp||In constructor ‘BoardData::BoardData(int, int)’:|
BoardData.cpp|4|error: expression list treated as compound expression in mem-initializer [-fpermissive]|
BoardData.cpp|4|warning: left operand of comma operator has no effect [-Wunused-value]|
BoardData.cpp|4|error: cannot convert ‘std::vector<int>’ to ‘int’ in initialization|
||=== Build finished: 4 errors, 1 warnings ===|

Answer 1

std::vector< std:vector<int> > m_field;

should be

std::vector< std::vector<int> > m_field;
                 ^

Not a very helpful error message, I agree...

Answer 2

Rather than fixing this compilation error, you should tweak your design. A rectangular array should not usually be stored as a vector of vectors, but rather as a single vector which you will index in a 2D fashion. You can use an existing library like Boost.Matrix for this, or implement it yourself:

m_field(rows*cols) // initialize
m_field[row + rows*col] // index

You can provide your own methods to index by row and column for safety. This will in the end be more efficient than the vector-of-vectors approach.