C ++ 17默认模板参数:没有参数列表的模板名称的无效使用 [英] C++17 Default template arguments: invalid use of template-name without an argument list
问题描述
使用C ++ 17,您可以像下面的示例一样在main中进行类模板参数推导:
With C++17, you can do class template argument deduction in main like in the following example:
template<class T = int>
struct X{};
int main()
{
X myX;
}
为什么数据成员不允许模板自变量推导?
Why is template argument deduction not allowed for data members?
template<class T = int>
struct X{};
struct Y
{
X myX;
};
int main()
{
Y myY;
}
错误:在没有参数列表X myX的情况下无效地使用了模板名称"X";
推荐答案
我没有参与该决定,但是,我确实看到了允许该决定的一些问题.让我们假设以下代码:
I wasn't involved into the decision, however, I do see some problems in allowing it. Let's assume the following code:
template<class T = int>
struct X
{
X(T t = T{}) {}
};
这使您的变量仍然可用:
This makes your variable still OK:
int main()
{
Y myY;
}
但是,如果Y具有在单独的文件中实现的构造函数怎么办?
However, what if Y has a constructor that's implemented in a separate file?
struct Y
{
Y();
X myX{'a'};
};
Y::Y() : myX{0.0} {}
在这种情况下,我们是否期望 myX
是 X< int>
或 X< double>
或 X< char>
?
Do we in this case expect myX
to be X<int>
or X<double>
or X<char>
?
我看到可能对此感到困惑.由于标准承诺者无法恢复其决定,因此最好采取确定的一小步,看看人们是否需要它以及他们期望发生什么.
I can see that there can be confusion about this. As the standards commite can't revert it's decisions, it's better to take a small step that's certain and see if people need it and what they expect to happen.
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