Question

我已经用c ++编写了一段时间，当我尝试编译这个简单的代码片段时我遇到了困难

#include "iostream"
using namespace std;

class A
{
 public:
  void f() { cout<<"f()\n"; }
};

int main()
{
// A a; //this works
 A *a = new A(); //this doesn't
 a.f(); // "f has not been declared"
 system("pause");
}

Answer 1

这是一个指针，所以请尝试：

a->f();

基本上，运算符.（用于访问对象的字段和方法）用于对象和引用，因此：

A a;
a.f();
A& ref = a;
ref.f();

如果您有指针类型，则必须先取消引用它以获取引用：

A* ptr = new A();
(*ptr).f();
ptr->f();

a->b表示法通常只是(*a).b的缩写。

关于智能指针的说明

operator->可以重载，智能指针特别使用它。当you're using smart pointers时，您还可以使用->来引用指向的对象：

auto ptr = make_unique<A>();
ptr->f();

Answer 2

允许分析。

#include <iostream>   // not #include "iostream"
using namespace std;  // in this case okay, but never do that in header files

class A
{
 public:
  void f() { cout<<"f()\n"; }
};

int main()
{
 /*
 // A a; //this works
 A *a = new A(); //this doesn't
 a.f(); // "f has not been declared"
 */ // below


 // system("pause");  <-- Don't do this. It is non-portable code. I guess your 
 //                       teacher told you this?
 //                       Better: In your IDE there is prolly an option somewhere
 //                               to not close the terminal/console-window.
 //                       If you compile on a CLI, it is not needed at all.
}

作为一般建议：

0) Prefer automatic variables
  int a;
  MyClass myInstance;
  std::vector<int> myIntVector;

1) If you need data sharing on big objects down 
   the call hierarchy, prefer references:

  void foo (std::vector<int> const &input) {...}
  void bar () { 
       std::vector<int> something;
       ...
       foo (something);
  }


2) If you need data sharing up the call hierarchy, prefer smart-pointers
   that automatically manage deletion and reference counting.

3) If you need an array, use std::vector<> instead in most cases.
   std::vector<> is ought to be the one default container.

4) I've yet to find a good reason for blank pointers.

   -> Hard to get right exception safe

       class Foo {
           Foo () : a(new int[512]), b(new int[512]) {}
           ~Foo() {
               delete [] b;
               delete [] a;
           }
       };

       -> if the second new[] fails, Foo leaks memory, because the
          destructor is never called. Avoid this easily by using 
          one of the standard containers, like std::vector, or
          smart-pointers.

根据经验法则：如果您需要自己管理内存，通常会有一位优先级经理或替代方案，符合RAII原则。

Answer 3

摘要：而不是a.f();应该是a->f();

在主要内容中，您已将 a 定义为指向 A 的对象的指针，因此您可以使用->访问函数操作

替代，但不太可读的方式是(*a).f()

a.f()可能已用于访问f（），如果 a 被声明为： A a;

Answer 4

a是一个指针。您需要使用->，而不是.

表达式必须具有类类型

4 个答案:

关于智能指针的说明