使用数组作为映射值:无法看到错误

Question

我正在尝试创建一个映射,其中键是一个int,值是一个数组

int red[3]  = {1,0,0};
int green[3] = {0,1,0};
int blue[3]     = {0,0,1};

    std::map<int, int[3]> colours;

colours.insert(std::pair<int,int[3]>(GLUT_LEFT_BUTTON,red)); //THIS IS LINE 24 !
colours.insert(std::pair<int,int[3]>(GLUT_MIDDLE_BUTTON,blue));
colours.insert(std::pair<int,int[3]>(GLUT_RIGHT_BUTTON,green));

但是,当我尝试编译此代码时,我收到以下错误.

g++ (Ubuntu 4.4.1-4ubuntu8) 4.4.1

 In file included from /usr/include/c++/4.4/bits/stl_algobase.h:66,
                 from /usr/include/c++/4.4/bits/stl_tree.h:62,
                 from /usr/include/c++/4.4/map:60,
                 from ../src/utils.cpp:9:
/usr/include/c++/4.4/bits/stl_pair.h: In constructor ‘std::pair<_T1, _T2>::pair(const _T1&, const _T2&) [with _T1 = int, _T2 = int [3]]’:
../src/utils.cpp:24:   instantiated from here
/usr/include/c++/4.4/bits/stl_pair.h:84: error: array used as initializer
/usr/include/c++/4.4/bits/stl_pair.h: In constructor ‘std::pair<_T1, _T2>::pair(const std::pair<_U1, _U2>&) [with _U1 = int, _U2 = int [3], _T1 = const int, _T2 = int [3]]’:
../src/utils.cpp:24:   instantiated from here
/usr/include/c++/4.4/bits/stl_pair.h:101: error: array used as initializer
In file included from /usr/include/c++/4.4/map:61,
                 from ../src/utils.cpp:9:
/usr/include/c++/4.4/bits/stl_map.h: In member function ‘_Tp& std::map<_Key, _Tp, _Compare, _Alloc>::operator[](const _Key&) [with _Key = int, _Tp = int [3], _Compare = std::less<int>, _Alloc = std::allocator<std::pair<const int, int [3]> >]’:
../src/utils.cpp:30:   instantiated from here
/usr/include/c++/4.4/bits/stl_map.h:450: error: conversion from ‘int’ to non-scalar type ‘int [3]’ requested
make: *** [src/utils.o] Error 1

我真的看不出错误在哪里.或者即使出现错误.

Answer 1

您不能像这样按值复制数组.

以下是几种解决方案,但我建议#4满足您的需求:

1)使用std::vector而不是数组

2)使用指向3个元素数组的指针映射.

int red[3]  = {1,0,0};
int green[3] = {0,1,0};
int blue[3]     = {0,0,1};
std::map<int,int(*)[3]> colours;
colours.insert(std::pair<int,int(*)[3]>(GLUT_LEFT_BUTTON,&red));
colours.insert(std::pair<int,int(*)[3]>(GLUT_MIDDLE_BUTTON,&blue));
colours.insert(std::pair<int,int(*)[3]>(GLUT_RIGHT_BUTTON,&green));
//Watch out for scope here, you may need to create the arrays on the heap.

3)使用boost元组而不是3个元素的数组.

4)不使用数组而是创建一个带有3个元素的新结构.制作地图.或者将数组包装在一个结构中,这也将起作用.

struct Triple
{
    int color[3];
};

 //Later in code
Tripple red = {1, 0, 0}, green = {0, 1, 0}, blue = {0, 0, 1};
std::map<int,Triple> colours;
colours.insert(std::pair<int,Triple>(GLUT_LEFT_BUTTON,red));
colours.insert(std::pair<int,Triple>(GLUT_MIDDLE_BUTTON,blue));
colours.insert(std::pair<int,Triple>(GLUT_RIGHT_BUTTON,green));

Answer 2

数组不是C++中的第一类构造.它们不是,Copy Constructible也不Assignable是对价值的要求std::map.你可以使用boost::array或std::vector.

Answer 3

使用std :: tr1 :: array.

typedef std::tr1::array<int, 3> Triple;
Triple red   = {1, 0, 0};
Triple green = {0, 1, 0};
Triple blue  = {0, 0, 1};
std::map<int, Triple> colours;
colours.insert(std::make_pair(GLUT_LEFT_BUTTON,   red));
colours.insert(std::make_pair(GLUT_MIDDLE_BUTTON, blue));
colours.insert(std::make_pair(GLUT_RIGHT_BUTTON,  green));

Answer 4

不要映射到int [],而是映射到int*,如下所示:

#include <iostream>
#include <map>
using namespace std;
int main(){
    std::map<int,int*> colors;
    int red[] = {3,7,9};
    colors[52] = red;
    cout << colors[52][1];  //prints 7
    colors[52][1] = 11;
    cout << colors[52][1];  //prints 11
    return 0;
}