为什么复制构造函数会调用其他类？默认构造函数？
我想知道为什么会出现这样的错误。
no matching function for call to 'Foo::Foo()'
在复制构造函数的代码中？假设 Foo 只是一个具有普通字段的对象（没有动态分配的内存等），并且它定义的唯一构造函数是一个带有一个参数的构造函数。
我什至不知道需要考虑构造函数。如果代码显示类似
bar = thing.bar; //
并且 bar 是 Foo 类型，并且具有上述规范，那么它不应该只生成一个浅拷贝并使用它来完成吗？为什么需要调用默认构造函数？
I was wondering why an error like this would occur.
no matching function for call to 'Foo::Foo()'
in code for a copy constructor? Assume Foo is just an object with normal fields ( no dynamically allocated memory, etc. ), and the only constructor it has defined is a constructor that takes one argument.
I didn't even know the constructor needed to be considered though. If the code says something like
bar = thing.bar; //
and bar is of Foo type, with the specifications described above, shouldn't it just generate a shallow copy and be done with it? Why does a default constructor need to be invoked?
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听起来您已经定义了复制构造函数，而没有定义任何其他构造函数。
一旦显式声明构造函数，编译器就不再为您提供默认构造函数。因此，您不再有一种机制来首先构造该类的对象（因此无法复制它）。
It sounds like you've defined the copy constructor without defining any other constructor.
Once you declare an constructor explicitly, the compiler no longer provides a default constructor for you. Consequently, you no longer have a mechanism to construct an object of the class in the first place (and therefore wouldn't be able to copy it).
如果，正如你所说，你正在做“
大概是在类的复制构造函数的 body 中的事情 - 所以
bar字段会使用其类的默认构造函数进行初始化首先，然后使用该类的赋值运算符来执行此语句。如果bar的类只有一个复制构造函数，没有默认构造函数，则需要添加一个bar(thing.bar)。子句 before 类的复制构造函数打开{并删除该赋值（无论如何通常都是一个好主意，但在“无默认构造函数”条件下是强制性的）。If, as you say, you're doing "something like
it's presumably in the body of your class's copy ctor -- so the
barfield gets initialized with its class's default ctor first, then uses that class's assignment operator for this statement. Ifbar's class only has a copy ctor, no default ctor, you'll need to add abar(thing.bar)clause before your class's copy ctor opening{and remove that assignment (generally a good idea anyway, but mandatory under the "no default ctor" condition).如果您没有定义构造函数，编译器将生成默认构造函数，但是如果您定义了构造函数（如复制构造函数），编译器将不会生成默认构造函数默认构造函数，因此您也需要定义该构造函数。
If you don't define a constructor, the compiler will generate a default constructor, however if you do define a constructor (Like a copy constructor) the compiler won't generate the default constructor, so you need to define that constructor too.