I tried to do
我试着这样做
MyClass& x;
x = MyClass(a,b,c);
But C++ won't let me do so because it thinks that x is uninitialized at the beginning.
但是C ++不会让我这样做,因为它认为x在开始时是未初始化的。
So I tried to do
所以我试着这样做
MyClass& x = MyClass(a,b,c);
But got error saying invalid initialization of non-const reference of type 'MyClass&' from an rvalue of type 'MyClass'
但得到的错误是从'MyClass'类型的右值开始无效初始化'MyClass&'类型的非const引用
What's wrong with it? It seems that I simply can't do anything now. How do I get around the initialization issue?
它出什么问题了?看来我现在根本无能为力。如何解决初始化问题?
2 个解决方案
#1
6
A reference must refer to an already-existing object. So you need to have an object first before you can refer to it.
引用必须引用已存在的对象。所以你需要先拥有一个对象才能引用它。
MyClass y = MyClass(a,b,c);
MyClass &x = y;
#2
9
An ordinary reference to non-const must be initialized with an lvalue expression (essentially an expression that refers to a memory location), e.g.
对非const的普通引用必须用左值表达式(本质上是指代存储器位置的表达式)初始化,例如,
MyClass o{ a, b, c };
MyClass& r = o;
If the reference is to const, or if it is an rvalue reference (denoted by &&), then the initializer can instead be an rvalue expression, such as a temporary produced by a function invocation:
如果引用是const,或者它是rvalue引用(用&&表示),那么初始化器可以是rvalue表达式,例如函数调用产生的临时表达式:
MyClass const& rc = foo();
MyClass&& rr = foo();
In these cases, for a local reference the lifetime of the temporary is extended to the scope of the reference.
在这些情况下,对于本地引用,临时的生命周期将扩展到引用的范围。
And one special feature is that if the initializer produces a temporary of a derived class, it's that full derived class object whose lifetime is extended, i.e. there's no slicing to the class specified for the reference.
还有一个特殊功能是,如果初始化程序生成一个派生类的临时类,那就是完整的派生类对象,它的生命周期被扩展,即对于为引用指定的类没有切片。
More generally the reference can be bound to any part of a temporary, as long as that part has a compatible type, and this will extend the lifetime of the full temporary object.
更一般地,引用可以绑定到临时的任何部分,只要该部分具有兼容类型,这将延长完整临时对象的生命周期。
#1
6
A reference must refer to an already-existing object. So you need to have an object first before you can refer to it.
引用必须引用已存在的对象。所以你需要先拥有一个对象才能引用它。
MyClass y = MyClass(a,b,c);
MyClass &x = y;
#2
9
An ordinary reference to non-const must be initialized with an lvalue expression (essentially an expression that refers to a memory location), e.g.
对非const的普通引用必须用左值表达式(本质上是指代存储器位置的表达式)初始化,例如,
MyClass o{ a, b, c };
MyClass& r = o;
If the reference is to const, or if it is an rvalue reference (denoted by &&), then the initializer can instead be an rvalue expression, such as a temporary produced by a function invocation:
如果引用是const,或者它是rvalue引用(用&&表示),那么初始化器可以是rvalue表达式,例如函数调用产生的临时表达式:
MyClass const& rc = foo();
MyClass&& rr = foo();
In these cases, for a local reference the lifetime of the temporary is extended to the scope of the reference.
在这些情况下,对于本地引用,临时的生命周期将扩展到引用的范围。
And one special feature is that if the initializer produces a temporary of a derived class, it's that full derived class object whose lifetime is extended, i.e. there's no slicing to the class specified for the reference.
还有一个特殊功能是,如果初始化程序生成一个派生类的临时类,那就是完整的派生类对象,它的生命周期被扩展,即对于为引用指定的类没有切片。
More generally the reference can be bound to any part of a temporary, as long as that part has a compatible type, and this will extend the lifetime of the full temporary object.
更一般地,引用可以绑定到临时的任何部分,只要该部分具有兼容类型,这将延长完整临时对象的生命周期。