c++ - 在 gcc 中将二维数组初始化为 0 时的值不正确

Question

#include <iostream>
using namespace std;

int main() {

    int rows = 10;
    int cols = 9;
    int opt[rows][cols] = {0};

         for (int i = 0; i < rows; ++i) {
            for (int j = 0; j < cols; ++j) {
                std::cout << opt[i][j] << " ";
            }
             std::cout << "\n";
         }

    return 0;
}

输出:

0 32767 1887606704 10943 232234400 32767 1874154647 10943 -1 
0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 
0 0 0 0 0 0 0 0 0 

我在 https://www.codechef.com/ide 中使用 gcc 6.3

我希望第一行全为零。不应该是这样吗？

编辑:我用 const 变量对行和列进行了测试，然后将其初始化为全零。我觉得这应该引发编译错误，而不是表现出这种不正确(并且有潜在危险)的行为。

Answer 1

如果我们查看 gcc 4.9 release notes看起来他们增加了对初始化 VLA 的支持，期望 VLA 在 future 的 C++ 版本中得到支持:

G++ supports C++1y variable length arrays. G++ has supported GNU/C99-style VLAs for a long time, but now additionally supports initializers and lambda capture by reference. In C++1y mode G++ will complain about VLA uses that are not permitted by the draft standard, such as forming a pointer to VLA type or applying sizeof to a VLA variable. Note that it now appears that VLAs will not be part of C++14, but will be part of a separate document and then perhaps C++17.

我们可以看到it live that before 4.9提示我们无法初始化 VLA

error: variable-sized object 'opt' may not be initialized  
     int opt[rows][cols] = {0};  
                             ^

但在 4.9.1 and after它停止提示，并且没有我们在最近的 versions 中看到的相同错误.

所以它看起来像一个回归。

请注意，clang 拒绝允许初始化 VLA (which they support as an extension)see a live example .从 C99 does not allow initialization of VLA 开始，这是有道理的:

The type of the entity to be initialized shall be an array of unknown size or an object type that is not a variable length array type.

gcc 错误 69517

海合会 bug report :SEGV on a VLA with excess initializer elements有一条评论提供了有关此功能的一些背景信息:

(In reply to Jakub Jelinek from comment #16)

The bug here is in G++ accepting a VLA initializer with more elements than there is room for in the VLA, and then trashing the stack at runtime with the extra elements. It is a regression with respect to GCC 4.9.3 which implements C++ VLAs as specified in n3639 (http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3639.html). This is documented in GCC 4.9 changes (https://gcc.gnu.org/gcc-4.9/changes.html) which highlights the feature using the following example:

  void f(int n) {
    int a[n] = { 1, 2, 3 }; // throws std::bad_array_length if n < 3
    ...

VLAs were subsequently removed from C++, and also partially (but not completely) removed from G++, which causes C++ programs developed and tested with G++ 4.9 to break when ported to a later version.

C++ VLAs will be safer to use with the patch referenced in comment #9. It patch had to be reverted from GCC 6.0 because it caused problems in Java. Java has been removed and I plan/hope to resubmit the patch for GCC 8. (I wanted to do it for GCC 7 but didn't get to it.)