[英]C++17 Default template arguments: invalid use of template-name without an argument list
With C++17, you can do class template argument deduction in main like in the following example:使用 C++17,您可以在 main 中进行类模板参数推导,如下例所示:
template<class T = int>
struct X{};
int main()
{
X myX;
}
Why is template argument deduction not allowed for data members?为什么数据成员不允许模板参数推导?
template<class T = int>
struct X{};
struct Y
{
X myX;
};
int main()
{
Y myY;
}
error: invalid use of template-name 'X' without an argument list X myX;错误:在没有参数列表 X myX 的情况下无效使用模板名称“X”;
I wasn't involved into the decision, however, I do see some problems in allowing it.我没有参与这个决定,但是,我确实看到允许它存在一些问题。 Let's assume the following code:让我们假设以下代码:
template<class T = int>
struct X
{
X(T t = T{}) {}
};
This makes your variable still OK:这使您的变量仍然可以:
int main()
{
Y myY;
}
However, what if Y has a constructor that's implemented in a separate file?但是,如果 Y 有一个在单独文件中实现的构造函数呢?
struct Y
{
Y();
X myX{'a'};
};
Y::Y() : myX{0.0} {}
Do we in this case expect myX
to be X<int>
or X<double>
or X<char>
?在这种情况下,我们是否希望myX
是X<int>
或X<double>
或X<char>
?
I can see that there can be confusion about this.我可以看到对此可能存在混淆。 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.由于标准委员会无法恢复其决定,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的事情。
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