问题描述
我很想知道为什么类的数据成员不能使用 () 语法初始化?考虑以下示例:
I am curious to know that why class' data members can't be initialized using () syntax? Consider following example:
#include <iostream>
class test
{
public:
void fun()
{
int a(3);
std::cout<<a<<'
';
}
private:
int s(3); // Compiler error why???
};
int main()
{
test t;
t.fun();
return 0;
}
程序编译失败&给出以下错误.
The program fails in compilation & gives following errors.
11 9 [Error] expected identifier before numeric constant
11 9 [Error] expected ',' or '...' before numeric constant
为什么?是什么原因?C++ 标准对类数据成员的初始化有何规定?非常感谢您的帮助.谢谢
Why? What is the reason? What the C++ standard says about initialization of class data members? Your help is greatly appreciated. Thanks
推荐答案
引入该功能的早期提案解释说 这是为了避免解析问题.
这里只是其中的一个例子:
Here's just one of the examples presented therein:
不幸的是，这使得( expression-list )"形式的初始化器在解析声明时变得模棱两可:
Unfortunately, this makes initializers of the "
(expression-list)" form ambiguous at the time that the declaration is being parsed:
struct S {
int i(x); // data member with initializer
// ...
static int x;
};
struct T {
int i(x); // member function declaration
// ...
typedef int x;
};
一种可能的解决方案是依赖于现有规则，即如果声明可以是对象或函数，那么它就是函数:
One possible solution is to rely on the existing rule that, if a declaration could be an object or a function, then it’s a function:
struct S {
int i(j); // ill-formed...parsed as a member function,
// type j looked up but not found
// ...
static int j;
};
类似的解决方案是应用另一个现有规则，目前仅在模板中使用，如果 T 可以是类型或其他东西，那么它就是其他东西；如果我们真的指的是类型，我们可以使用typename":
A similar solution would be to apply another existing rule, currently used only in templates, that if T could be a type or something else, then it’s something else; and we can use "typename" if we really mean a type:
struct S {
int i(x); // unabmiguously a data member
int j(typename y); // unabmiguously a member function
};
这两种解决方案都引入了许多用户可能会误解的微妙之处(comp.lang.c++ 上关于为什么int i();"在块范围内的许多问题证明了这一点没有声明默认初始化的 int).
Both of those solutions introduce subtleties that are likely to be misunderstood by many users (as evidenced by the many questions on comp.lang.c++ about why "int i();" at block scope doesn’t declare a default-initialized int).
本文提出的解决方案是只允许= initializer-clause"和{ initializer"的初始化器-list }"表格.这解决了大多数情况下的歧义问题.[..]
The solution proposed in this paper is to allow only initializers of the "= initializer-clause" and "{ initializer-list }" forms. That solves the ambiguity problem in most cases. [..]
这篇关于为什么不能通过直接初始化语法来初始化类数据成员?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，WP2