错误 "invalid types ... for array subscript" 是什么意思？

Question

我可以在 SO 上看到对这个错误的几个引用，虽然所有答案似乎都解决了原始编译错误，但其中 none 解释了错误的真正含义。

我正在编译我的 cpp 文件：g++ -Wall -std=c++11 myfile.cpp，并得到以下错误：

myfile.cpp: In function ‘void GenerateMatrix(uint8_t**, uint8_t)’:
myfile.cpp:32:39: error: invalid types ‘uint8_t {aka unsigned char}[uint8_t {aka unsigned char}]’ for array subscript
    std::cout << ", " << (*matrix)[i][j];

我的代码：

#include <iostream>

//// populates an n x n matrix.
//// @return the matrix 
void GenerateMatrix(uint8_t** matrix, uint8_t n)
{

    *matrix = (uint8_t*)malloc(n * n);

    uint8_t* pc = *matrix;
    for(uint8_t i = 0; i < n; i++)
    {
        for(uint8_t j = 0; j < n; j++)
        {
            *pc++ = i+j;
        }
    }

    for(uint8_t i = 0; i < n; i++)
    {
        for(uint8_t j = 0; j < n; j++)
        {
            std::cout << ", " << (*matrix)[i][j];
        }
        std::cout << "\n";
    }
}


int main()
{
    uint8_t* matrix = nullptr;
    uint8_t n = 10;
    GenerateMatrix(&matrix, n);
    return 0;
}

我尝试将第二个 for 循环中的 i 和 j 更改为 int。那给了我一个类似的错误，但这次投诉是关于invalid types ‘uint8_t {aka unsigned char}[int]’，我还是none更聪明。

谁能帮我理解这个错误？

Answer 1

void generateMatrix(uint8_t** matrix, uint8_t n)
//                         ^^
{
    (*matrix) // type is uint8_t*
    [i]       // type is uint8_t
    [j];      // ???
}

你实际做的相当于：

uint8_t n = 10;
n[12] = 7;   // no index ('subscript'!) applicable to raw unsigned char
             // or with compiler words, the unsigned char is an invalid
             // type for this operation to be applied on...

同样的消息也可能出现在另一个方向：

class C { }; // note that there's no cast operator to some integral type provided!

int array[7];
C c;
array[c]; // just that this time it's the other operand that has invalid type...

错误 "invalid types ... for array subscript" 是什么意思？

What does the error "invalid types ... for array subscript" mean?

c++

arrays

g++