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

Question

我可以在SO上看到对此错误的多次引用，尽管答案似乎都解决了原始编译错误，但没有一个解释错误的真正含义。

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;
}

我尝试更改i并在第二个循环中j以int.这给了我一个类似的错误，但这次的抱怨是关于invalid types ‘uint8_t {aka unsigned char}[int]’，我仍然不明智。

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...

在另一个方向上也可能出现相同的消息：

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...