构造函数不返回可用对象 [英] Constructor not returning usable object

问题描述

我的构造函数有问题，它无法按预期工作.

I have a problem with the constructor, which is not working as I'd expect.

如果我尝试像这样初始化我的类，它将起作用并且我得到一个可用的对象:

If I try to initialize my class like that, it will work and I get a usable object:

vector<float> v;
MyClass<2> a(v);

但是，如果我尝试构建如下所示的类(应该是等效的)，结果将是非常意外的.编译或运行程序时，没有错误消息/警告.但是，如果您尝试在某个地方使用此变量并调用其方法(例如a.doSomething())，它将崩溃.

However, if I try to build a class like below (which should be equivalent) the results are quite unexpected. There is no error message/warning when compiling or running the program. But if you try to use this variable a somewhere and call its methods (for example a.doSomething()), it will crash.

我在构造函数中放入了一些代码，以通知我是否使用了它.事实证明，在这种情况下，构造函数内部实际上没有执行任何代码.

I put some code inside the constructor to notify me if it is used. It turned out that no code inside the constructor was actually executed in this case.

MyClass<2> a(vector<float>());

所以我想知道为什么会这样吗?第二条声明是否违法?

So I am wondering why this is happening? Is the 2nd declaration illegal?

我将发布该类的一些代码

I will post some code of the class

template <int x>
class MyClass {
public:
    vector<float> v;
    MyClass<x>(vector<float> v1) {
      v = v1;
    }

};

推荐答案

MyClass<2> a(vector<float>());

这不是变量声明.它是一个名为 a 的函数的声明，该函数返回一个 MyClass< 2> 对象，并以指向不带任何参数并返回一个a的函数的指针"作为参数. vector&float> ."令人困惑?是的.这就是所谓的最烦人的解析".

This is not a variable declaration. It is the declaration of a function named a that returns a MyClass<2> object and takes as an argument a "pointer to a function that takes no arguments and returns a vector<float>." Confusing? Yes. This is what is referred to as the "most vexing parse."

您需要额外的括号:

MyClass<2> a((vector<float>()));
             ^               ^

或者，您可以使用副本初始化:

Or, you can use copy initialization:

MyClass<2> a = MyClass<2>(vector<float>());

或者，由于您的构造函数不是 explicit ，因此您可以使用:

Or, since your constructor isn't explicit, you could use:

MyClass<2> a = vector<float>();

(尽管，除非您要使 vector< float> 对象隐式转换为 MyClass< N> 对象，否则您可能希望使此构造函数成为明确的.)

(Though, unless you mean for vector<float> objects to be implicitly convertible to MyClass<N> objects, you probably want to make this constructor explicit.)

一个好的编译器应该警告您这种事情.Visual C ++警告:

A good compiler should warn you about this sort of thing. Visual C++ warns:

警告C4930:' MyClass< x>a(std :: vector< _Ty>(__cdecl *)(void))':未调用原型函数(是否打算使用变量定义?)

warning C4930: 'MyClass<x> a(std::vector<_Ty> (__cdecl *)(void))': prototyped function not called (was a variable definition intended?)

C警告:

警告:括号已明确地作为函数声明符

warning: parentheses were disambiguated as a function declarator

MyClass<2> a(vector<float>());
            ^~~~~~~~~~~~~~~~~

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