C ++在类中声明数组并在类中声明2d数组

Question

I'm new with using classes and I encountered a problem while delcaring an array into a class. I want to initialize a char array for text limited to 50 characters and then replace the text with a function.

#ifndef MAP_H
#define MAP_H
#include "Sprite.h"
#include <SFML/Graphics.hpp> 
#include <iostream>

class Map : public sprite
{
private:
    char mapname[50];
    int columnnumber;
    int linenumber;
    char casestatematricia[];

public:
    void setmapname(char newmapname[50]);
    void battlespace(int column, int line);
    void setcasevalue(int col, int line, char value);
    void printcasematricia();

};


#endif

By the way I could initialize my 2d array like that

char casestatematricia[][];

I want later to make this 2d array dynamic where I enter a column number and a line number like that

casestatematricia[linenumber][columnnumber]

to create a battlefield.

this is the cpp code so that you have an idea of what I want to do.

#include "Map.h"
#include <SFML/Graphics.hpp> 
#include <iostream>

using namespace sf;

void Map::setmapname(char newmapname[50])
{
    this->mapname = newmapname;
}
void Map::battlespace(int column, int line)
{

}
void Map::setcasevalue(int col, int line, char value)
{

}
void Map::printcasematricia()
{

}

thank you in advance.

Answer 1

Consider following common practice on this one. Most (eg numerical) libraries don't use 2D arrays inside classes. They use dynamically allocated 1D arrays and overload the () or [] operator to access the right elements in a 2D-like fashion. So on the outside you never can tell that you're actually dealing with consecutive storage, it looks like a 2D array. In this way arrays are easier to resize, more efficient to store, transpose and reshape.

Answer 2

Just a proposition for your problem:

class Map : public sprite
{
private:
    std::string mapname;
    int columnnumber;
    int linenumber;
    std::vector<char> casestatematricia;

    static constexpr std::size_t maxRow = 50;
    static constexpr std::size_t maxCol = 50; 

public:
    Map():
        casestatematricia(maxRow * maxCol, 0)
    {}
    void setmapname(std::string newmapname)
    {
        if (newmapname.size() > 50)
        {
            // Manage error if you really need no more 50 characters..
            // Or just troncate when you serialize!
        }
        mapname = newmapname;
    }

    void battlespace(int col, int row);
    void setcasevalue(int col, int row, char value)
    {
        // check that col and line are between 0 and max{Row|Column} - 1
        casestatematricia[row * maxRow + col] = value;
    }

    void printcasematricia()
    {
        for (std::size_t row = 0; row < maxRow; ++row)
        {
            for (std::size_t col = 0; col < maxCol; ++col)
            {
                char currentCell = casestatematricia[row * maxRow + col];
            }
        }
    }
};

For access to 1D array like a 2D array, take a look at Access a 1D array as a 2D array in C++ .

When you think about serialization, I guess you want to save it to a file. Just a advice: don't store raw memory to a file just to "save" time when your relaunch your soft. You just have a non portable solution! And seriously, with power of your computer, you don't have to be worry about time to load from file!

I propose you to add 2 methods in your class to save Map into file

void dump(std::ostream &os)
{
    os << mapname << "\n";
    std::size_t currentRow = 0;
    for(auto c: casestatematricia)
    {
        os << static_cast<int>(c) << " ";
        ++currentRow;

        if (currentRow >= maxRow)
        {
            currentRow = 0;
            os << "\n";
        }
    }
}

void load(std::istream &is)
{
    std::string line;

    std::getline(is, line);
    mapname = line;

    std::size_t current_cell = 0;
    while(std::getline(is, line))
    {
        std::istringstream is(line);
        while(!is.eof())
        {
            char c;

            is >> c;
            casestatematricia[current_cell] = c;

            ++current_cell;
        }
    }
}

This solution is only given for example. They doesn't manage error and I have choose to store it in ASCII in file. You can change to store in binary, but, don't use direct write of raw memory. You can take a look at C - serialization techniques (just have to translate to C++). But please, don't use memcpy or similar technique to serialize

Answer 3

I hope I get this right. You have two questions. You want know how to assign the value of char mapname[50]; via void setmapname(char newmapname[50]); . And you want to know how to create a dynamic size 2D array.

I hope you are comfortable with pointers because in both cases, you need it.

For the first question, I would like to first correct your understanding of void setmapname(char newmapname[50]); . C++ functions do not take in array. It take in the pointer to the array. So it is as good as writing void setmapname(char *newmapname); . For better understanding, go to Passing Arrays to Function in C++

With that, I am going to change the function to read in the length of the new map name. And to assign mapname , just use a loop to copy each of the char.

void setmapname(char *newmapname, int length) {
    // ensure that the string passing in is not 
    // more that what mapname can hold.
    length = length < 50 ? length : 50;

    // loop each value and assign one by one. 
    for(int i = 0; i < length; ++i) {
        mapname[i] = newmapname[i];
    }
}

For the second question, you can use vector like what was proposed by Garf365 need to use but I prefer to just use pointer and I will use 1D array to represent 2d battlefield. (You can read the link Garf365 provide).

// Declare like this
char *casestatematricia; // remember to initialize this to 0.

// Create the battlefield
void Map::battlespace(int column, int line) {

    columnnumber = column;
    linenumber = line;

    // Clear the previous battlefield.
    clearspace();

    // Creating the battlefield
    casestatematricia = new char[column * line];

    // initialise casestatematricia...
}

// Call this after you done using the battlefield
void Map::clearspace() {
    if (!casestatematricia) return;

    delete [] casestatematricia;
    casestatematricia = 0;
}

Just remember to call clearspace() when you are no longer using it.

Just for your benefit, this is how you create a dynamic size 2D array

// Declare like this
char **casestatematricia; // remember to initialize this to 0.

// Create the battlefield
void Map::battlespace(int column, int line) {

    columnnumber = column;
    linenumber = line;

    // Clear the previous battlefield.
    clearspace();

    // Creating the battlefield
    casestatematricia = new char*[column];
    for (int i = 0; i < column; ++i) {
        casestatematricia[i] = new char[line];
    }

    // initialise casestatematricia...
}

// Call this after you done using the battlefield
void Map::clearspace() {
    if (!casestatematricia) return;

    for(int i = 0; i < columnnumber; ++i) {
        delete [] casestatematricia[i];
    }

    delete [][] casestatematricia;
    casestatematricia = 0;
} 

Hope this help.

PS: If you need to serialize the string, you can to use pascal string format so that you can support string with variable length. eg "11hello world", or "3foo".

Answer 4

Replace:

char mapname[50];

with

char *mapname;

In constructor of your class allocate memory:

mapname = new char[50];

Then replace:

void Map::setmapname(char newmapname[50])
{
    this->mapname = newmapname;
}

with:

void Map::setmapname(char *newmapname, size_t size)
{
    memcpy(mapname,newmapname,size);
}

EDIT: As pointed out in comments I forgot destructor: In destructor free the memory:

if(mapname)
delete [] mapname;

As the user who posted this question he is new to classes so also to C++, that is why there are no containers and things like that in this answear.