c++ - 当我聚合初始化一个数组而 gcc 没有时，Clang 警告我

Question

当我用 CLANG 编译以下代码时:

#include <iostream>
#include <array>
#include <algorithm>
#include <functional>

int main() {
  std::array<int, 2> a = {1, 2};
  std::array<int, 2> b = {2, 1};
  std::array<int, 2> c;
  std::transform(a.begin(), a.end(), b.begin(), c.begin(), std::multiplies<int>());
  for(auto &&i : c) std::cout << i << " ";
  std::cout << std::endl;
}

通过发出命令:

clang++ -std=c++14 -O2 -Wall -pedantic -pthread main.cpp

它发出警告:

warning: suggest braces around initialization of subobject [-Wmissing-braces]

CLANG DEMO

但是，GCC 编译这个程序时根本没有发出警告。

GCC DEMO

问:

1. 哪个编译器是正确的？
2. Clangs 警告我的原因是什么？

Answer 1

在某些情况下，大括号可以省略。这是其中一个案例。用于初始化 a 和 b 的最外层大括号是可选的。无论哪种方式在语法上都是正确的——但只包含它们会更清楚。 Clang 只是就此警告您(警告，而非错误)——这是一个完全有效的警告。而作为 chris , 指出，对于 -Wmissing-braces，gcc 会发出相同的警告。最终，两个编译器都接受了正确的代码；它毕竟是一个有效的程序。这才是最重要的。

来自 [dcl.init.aggr]:

Braces can be elided in an initializer-list as follows. If the initializer-list begins with a left brace, then the succeeding comma-separated list of initializer-clauses initializes the members of a subaggregate; it is erroneous for there to be more initializer-clauses than members. If, however, the initializer-list for a subaggregate does not begin with a left brace, then only enough initializer-clauses from the list are taken to initialize the members of the subaggregate; any remaining initializer-clauses are left to initialize the next member of the aggregate of which the current subaggregate is a member. [ Example:

float y[4][3] = {
    { 1, 3, 5 },
    { 2, 4, 6 },
    { 3, 5, 7 },
};

is a completely-braced initialization: 1, 3, and 5 initialize the first row of the array y[0], namely y[0][0], y[0][1], and y[0][2]. Likewise the next two lines initialize y[1] and y[2]. The initializer ends early and therefore y[3]s elements are initialized as if explicitly initialized with an expression of the form float(), that is, are initialized with 0.0. In the following example, braces in the initializer-list are elided; however the initializer-list has the same effect as the completely-braced initializer-list of the above example,

float y[4][3] = {
    1, 3, 5, 2, 4, 6, 3, 5, 7
};

The initializer for y begins with a left brace, but the one for y[0] does not, therefore three elements from the list are used. Likewise the next three are taken successively for y[1] and y[2]. —end example ]