请考虑以下代码：
struct Foo
{
int x,y;
Foo() = default;
Foo(const Foo&) = delete;
Foo& operator=(const Foo&) = delete;
};
int main()
{
Foo f1 {1,2};
Foo f2 = {1,2};
}
用clang编译没有错误：
$clang++ --version Apple LLVM version 4.2 (clang-425.0.28) (based on LLVM 3.2svn) Target: x86_64-apple-darwin12.4.0 Thread model: posix $clang++ -std=c++11 -stdlib=libc++ -pedantic t.cpp -o out ...builds and runs fine...
但是,compiling with g++ 4.8.1 through ideone gives errors：
prog.cpp: In function ‘int main()’:
prog.cpp:12:17: error: no matching function for call to ‘Foo::Foo(<brace-enclosed initializer list>)’
Foo f1 {1,2};
^
prog.cpp:12:17: note: candidate is:
prog.cpp:5:5: note: Foo::Foo()
Foo() = default;
^
prog.cpp:5:5: note: candidate expects 0 arguments,2 provided
prog.cpp:13:19: error: could not convert ‘{1,2}’ from ‘<brace-enclosed initializer list>’ to ‘Foo’
Foo f2 = {1,2};
^
如果我删除Foo(const Foo&)= delete;那么它在compiles fine在g 4.8.1.
解决方法
C 11 8.5.1 [dcl.init.aggr] p1定义聚合类型：
An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1),no brace-or-equal-initializers for non-static data members (9.2),no private or protected non-static data members (Clause 11),no base classes (Clause 10),and no virtual functions (10.3).
用户提供的是在8.4.2 [dcl.fct.def.default] p4中定义的：
… A special member function is user-provided if it is user-declared and not explicitly
defaulted or deleted on its first declaration.
Foo有两个用户声明的构造函数,它们都在其第一个声明中被明确地默认或删除,所以Foo是一个聚合.
海湾合作委员会错了.
