C++ - 返回对象的函数
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C++ - function that returns object
提问by Kevin Meredith
// Assume class definition for Cat is here.
Cat makeCat() {
Cat lady = new Cat("fluffy");
return lady;
}
int main (...) {
Cat molly = makeCat();
molly->eatFood();
return 0;
}
Will there be a "use after free"error on molly->eatFood()?
是否会出现“免费后使用”错误molly->eatFood()？
回答by Notinlist
Corrected your program and created an example implementation of class Cat:
更正了您的程序并创建了一个示例实现class Cat：
#include <iostream>
#include <string>
class Cat {
public:
Cat(const std::string& name_ = "Kitty")
: name(name_)
{
std::cout << "Cat " << name << " created." << std::endl;
}
~Cat(){
std::cout << "Cat " << name << " destroyed." << std::endl;
}
void eatFood(){
std::cout << "Food eaten by cat named " << name << "." << std::endl;
}
private:
std::string name;
};
Cat* makeCat1() {
return new Cat("Cat1");
}
Cat makeCat2() {
return Cat("Cat2");
}
int main (){
Cat kit = makeCat2();
kit.eatFood();
Cat *molly = makeCat1();
molly->eatFood();
delete molly;
return 0;
}
It will produce the output:
它将产生输出：
Cat Cat2 created.
Food eaten by cat named Cat2.
Cat Cat1 created.
Food eaten by cat named Cat1.
Cat Cat1 destroyed.
Cat Cat2 destroyed.
I suggest you learn a basic book about the C++ cover to cover before continuing.
我建议你在继续之前学习一本关于 C++ 的基础书籍。
回答by jwueller
new Cat("fluffy")creates a pointer. You will need to specify Cat*as return type. Since the object is created in heap, it will still be available after the function returned.
new Cat("fluffy")创建一个指针。您需要指定Cat*为返回类型。由于对象是在堆中创建的，所以函数返回后它仍然可用。
回答by Brian R. Bondy
There is no error as far as invalid use of memory other than a memory leak at the end of your program. If something is created on the heap (such as with new) then you need to call deleteon it to free it.
除了程序结束时的内存泄漏之外，就内存的无效使用而言，没有错误。如果在堆上创建了一些东西（比如 with new），那么你需要调用delete它来释放它。
You also have a lot of syntax errors, corrected below.
你也有很多语法错误，下面更正。
Cat* makeCat()
{
Cat *lady = new Cat("fluffy");
return lady;
}
int main (int argc, char** argv)
{
Cat* molly = makeCat();
molly->eatFood();
delete molly;//This was added
return 0;
}
回答by Starkey
Since ladyis created on the heap (with a new), it will not be destroyed when you exit the makeCatmethod. So the call on mollyis perfectly valid.
由于lady是在堆上创建的（带有new），因此退出makeCat方法时不会销毁它。所以调用molly是完全有效的。
BUT, you have a memory leak. You need to delete mollyafter using it (sometime in the future.) Since your program ends, this is not a big deal. In a larger program, this would be a very big deal.
但是，您有内存泄漏。molly使用后需要删除（以后的某个时间）。既然你的程序结束了，这没什么大不了的。在一个更大的程序中，这将是一个非常大的问题。
回答by koresh
If your complier supports c++11 you can use unique_ptr here:
如果您的编译器支持 c++11，您可以在此处使用 unique_ptr：
#include <iostream>
#include <memory>
using namespace std;
class Cat {
public:
Cat() {
cout << "Cat created" << endl;
}
~Cat() {
cout << "Cat destroyed" << endl;
}
void eatFood() {
cout << "Cat is eating food" << endl;
}
};
unique_ptr<Cat> makeCat() {
unique_ptr<Cat> lady( new Cat );
return lady;
}
int main () {
unique_ptr<Cat> molly = makeCat();
molly->eatFood();
return 0;
}
Now, you don't have to worry about deleting created object. It will be deleted as soon as molly pointer goes out of scope:
现在，您不必担心删除创建的对象。一旦 molly 指针超出范围，它将被删除：
Cat created
Cat is eating food
Cat destroyed
回答by pascal
The problem is not a "Use after Free"; more probably that you're not delete-ing the new instance.
问题不是“免费后使用”；更有可能的是您没有删除新实例。
回答by Eiko
I think it should be Cat *ladyand Cat *molly, but else it should be ok.
我认为应该是Cat *ladyand Cat *molly，否则应该没问题。
