如何从另一个构造函数调用 C++ 类构造函数
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How to call a C++ class Constructor from another Constructor
提问by WoodMath
I'm trying to create an object in C++ that requires multiple object constructors. Say Foo()and Foo(int)where Foo(int)then calls Foo(). The simplified code is written below:
我正在尝试用 C++ 创建一个需要多个对象构造函数的对象。说Foo()然后Foo(int)在哪里Foo(int)打电话Foo()。简化后的代码如下：
#include <iostream>
class Foo{
private:
int iX;
public:
void printX(string sLabel){
cout << sLabel << " : " << " Foo::iX = " << Foo::iX << endl;
};
void setX(int iX){
Foo::iX = iX;
Foo::printX("setX(void) Method");
};
Foo(){
Foo::iX = 1;
Foo::printX("Foo(void) Constructor");
};
Foo(int iX){
Foo::setX(iX);
Foo::printX("Foo(int) Constructor");
Foo::Foo();
Foo::printX("Foo(int) Constructor");
};
};
int main( int argc, char** argv ){
Foo bar(2);
return 0;
}
The output of which is
其中的输出是
setX(void) Method : Foo::iX = 2
Foo(int) Constructor : Foo::iX = 2
Foo(void) Constructor : Foo::iX = 1
Foo(int) Constructor : Foo::iX = 2
As the results indicate setXmethod works as expected. Foo::iXis equal to 2inside and outside of scope of that function.
结果表明setX方法按预期工作。Foo::iX等于2该函数的范围内外。
However when calling the Foo(void)constructor from within the Foo(int)constructor, Foo::iXstays equal to 1only within that constructor. As soon as it exits out that method, it reverts back to 2.
但是，当Foo(void)从Foo(int)构造函数中调用构造函数时，仅在该构造函数中Foo::iX保持等于1。一旦退出该方法，它就会恢复到2.
So my question is 2-fold:
所以我的问题是两方面的：
- Why does C++ behave this (a constructor cannot be called from within another constructor without values that were assigned)?
- How can you create multiple constructor signatures, but without redundant duplicate code doing the same thing?
- 为什么 C++ 会这样（一个构造函数不能在没有赋值的情况下从另一个构造函数中调用）？
- 如何创建多个构造函数签名，但没有多余的重复代码做同样的事情？
回答by songyuanyao
Foo::Foo();in Foo::Foo(int)is not invoking the default constructor on the current object as you expected. It just constructs a temporary Foo, which has nothing to with the current object.
Foo::Foo();inFoo::Foo(int)没有按预期调用当前对象的默认构造函数。它只是构造一个临时的Foo，与当前对象无关。
You can use delegating constructor(since C++11) like this:
您可以像这样使用委托构造函数（C++11 起）：
Foo(int iX) : Foo() {
// ...
};
Note that Foo::Foo()will be invoked in advance of the body of Foo::Foo(int)here.
请注意，Foo::Foo()将在Foo::Foo(int)此处的主体之前调用。
An alternative to avoid duplicated code is to use setX()as a common initialization method. (Or make a new one if not appropriate.)
避免重复代码的另一种方法是setX()用作通用初始化方法。（如果不合适，或者制作一个新的。）
Foo() {
setX(1);
// ...
};
Foo(int iX) {
setX(iX);
// ...
};
回答by R Sahu
If you are able to use a C++11 compiler, you can use delegating constructors.
如果您能够使用 C++11 编译器，则可以使用委托构造函数。
// Use Foo(int) to initialize the object when default constructor is used.
Foo() : Foo(1) {}
回答by Wiki Wang
A constructor CAN be called from within another constructor. You just forget to assign the return value.
可以从另一个构造函数中调用构造函数。您只是忘记分配返回值。
*this = Foo::Foo();
For question 2, you need delegating constructors.
对于问题 2，您需要委托构造函数。
Foo(int iX) : Foo() {
...
}
