如何修复隐式转换警告?

Question

我正在阅读C++如何编程第9版，尝试学习c ++，并尝试在第7章中使用骑士问题进行练习。

基于输出中的"64"，代码显然存在某种逻辑错误(我将自行修复)，但在执行此操作之前，我坚持理解以及如何处理编译器警告。

我将 board 声明为无符号 int 的二维数组，因为它只能包含值 0-7。同样，出于同样的原因，我将 for 循环中的变量声明为无符号 int。

我已经阅读了有关此的堆栈溢出的回复，但它们很快就超出了我的脑海。

#include <iostream>
#include <array>
#include <iomanip>
using namespace std;
void printArray(const array< array< unsigned int, 8 >, 8 >);
int main() {
array< array< unsigned int, 8 >, 8 > board = {};
array< int, 8 > horizontal = {{ 2, 1, -1, -2, -2, -1, 1, 2 }};
array< int, 8 > vertical = {{ -1, -2, -2, -1, 1, 2, 2, 1 }};
unsigned int currentRow = 0;
unsigned int currentCol = 0;
for (unsigned int i = 0; i < 64; i++) {
board[currentRow][currentCol] = i + 1;
for (unsigned int j = 0; j < 8; j++) {
if (currentRow + horizontal[j] >= 0 &&
currentRow + horizontal[j] < 8 &&
currentCol + vertical[j] >= 0 &&
currentCol + vertical[j] < 8 &&
board[currentRow + horizontal[j]][currentCol + vertical[j]] == 0
) {
currentRow = currentRow + horizontal[j];
currentCol = currentCol + vertical[j];
break;
}
}
}
printArray(board);
}
void printArray(const array< array< unsigned int, 8 >, 8 > myBoard) {
for ( unsigned int i = 0; i < 8; i++) {
for ( unsigned int j = 0; j < 8; j++) {
cout << '[' << setw(2) << setfill('0') << myBoard[i][j] << "] ";
}
cout << endl;
}
}

输出：

~/Programming/cpp/knight => make
rm -fr build/*
clang++ -std=c++11 -stdlib=libc++ -Weverything -Wno-c++98-compat src/main.cpp -o build/main -Isrc/
src/main.cpp:18:24: warning: implicit conversion changes signedness: 'value_type' (aka 'int') to 'unsigned int' [-Wsign-conversion]
if (currentRow + horizontal[j] >= 0 &&
~ ^~~~~~~~~~~~~
src/main.cpp:18:38: warning: comparison of unsigned expression >= 0 is always true [-Wtautological-compare]
if (currentRow + horizontal[j] >= 0 &&
~~~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~
src/main.cpp:19:22: warning: implicit conversion changes signedness: 'value_type' (aka 'int') to 'unsigned int' [-Wsign-conversion]
currentRow + horizontal[j] < 8 &&
~ ^~~~~~~~~~~~~
src/main.cpp:20:22: warning: implicit conversion changes signedness: 'value_type' (aka 'int') to 'unsigned int' [-Wsign-conversion]
currentCol + vertical[j] >= 0 &&
~ ^~~~~~~~~~~
src/main.cpp:20:34: warning: comparison of unsigned expression >= 0 is always true [-Wtautological-compare]
currentCol + vertical[j] >= 0 &&
~~~~~~~~~~~~~~~~~~~~~~~~ ^  ~
src/main.cpp:21:22: warning: implicit conversion changes signedness: 'value_type' (aka 'int') to 'unsigned int' [-Wsign-conversion]
currentCol + vertical[j] < 8 &&
~ ^~~~~~~~~~~
src/main.cpp:22:28: warning: implicit conversion changes signedness: 'value_type' (aka 'int') to 'unsigned int' [-Wsign-conversion]
board[currentRow + horizontal[j]][currentCol + vertical[j]] == 0
~ ^~~~~~~~~~~~~
src/main.cpp:22:56: warning: implicit conversion changes signedness: 'value_type' (aka 'int') to 'unsigned int' [-Wsign-conversion]
board[currentRow + horizontal[j]][currentCol + vertical[j]] == 0
~ ^~~~~~~~~~~
src/main.cpp:24:35: warning: implicit conversion changes signedness: 'value_type' (aka 'int') to 'unsigned int' [-Wsign-conversion]
currentRow = currentRow + horizontal[j];
~ ^~~~~~~~~~~~~
src/main.cpp:25:35: warning: implicit conversion changes signedness: 'value_type' (aka 'int') to 'unsigned int' [-Wsign-conversion]
currentCol = currentCol + vertical[j];
~ ^~~~~~~~~~~
10 warnings generated.
./build/main
[01] [24] [09] [00] [00] [26] [37] [00]
[10] [41] [02] [25] [36] [00] [00] [27]
[23] [08] [11] [40] [21] [38] [00] [00]
[64] [03] [22] [35] [00] [00] [28] [00]
[07] [12] [05] [20] [39] [34] [00] [00]
[04] [17] [14] [31] [00] [29] [00] [00]
[13] [06] [19] [16] [33] [00] [00] [00]
[18] [15] [32] [00] [30] [00] [00] [00]

Answer 1

编译器对你说，当你写的时候

currentRow + horizontal[j] >= 0

你添加一个带有unsigned int的signed int，所以signed int被转换成一个unsigned int，所以总和的结果是一个unsigned int。unsigned int始终大于或等于零。

所以这个测试总是true这可能不完全是你想要的。

同样的问题

currentCol + vertical[j] >= 0

更一般地说，对于signed/unsigned值的每一和。

一种选择是将currentRow和currentCol定义为(signed)int。

否则，您可以在将值添加到signed值时适当地转换unsigned

值

int{currentRow} + horizontal[j] >= 0