类的成员默认为private, struct的成员默认为public。

例如，程序1编译失败，程序2运行正常。

// Program 1
#include <stdio.h>
 
class Test {
    int x; // x is private
};

int main()
{
  Test t;
  t.x = 20; // compiler error because x is private
  getchar();
  return 0;
}

// Program 2
#include <stdio.h>
 
struct Test {
    int x; // x is public
};

int main()
{
  Test t;
  t.x = 20; // works fine because x is public
  getchar();
  return 0;
}

从类/结构派生结构时，基类/结构的默认访问说明符为public。派生类时，默认访问说明符为private。

例如，程序3编译失败，程序4工作正常。

// Program 3
#include <stdio.h>
 
class Base {
public:
    int x;
};
 
class Derived : Base { }; // is equivalent to class Derived : private Base {}
 
int main()
{
  Derived d;
  d.x = 20; // compiler error because inheritance is private
  getchar();
  return 0;
}

// Program 4
#include <stdio.h>
 
class Base {
public:
    int x;
};
 
struct Derived : Base { }; // is equivalent to struct Derived : public Base {}
 
int main()
{
  Derived d;
  d.x = 20; // works fine because inheritance is public
  getchar();
  return 0;
}

. In classes all the members by default are private but in structure members are public by default. There is no term like constructor and destructor for structs, but for class compiler creates default if you don't provide. Sizeof empty structure is 0 Bytes wer as Sizeof empty class is 1 Byte The struct default access type is public. A struct should typically be used for grouping data. The class default access type is private, and the default mode for inheritance is private. A class should be used for grouping data and methods that operate on that data. In short, the convention is to use struct when the purpose is to group data, and use classes when we require data abstraction and, perhaps inheritance. In C++ structures and classes are passed by value, unless explicitly de-referenced. In other languages classes and structures may have distinct semantics - ie. objects (instances of classes) may be passed by reference and structures may be passed by value. Note: There are comments associated with this question. See the discussion page to add to the conversation.

STRUCT是一种抽象数据类型，它根据结构规范划分给定的内存块。结构体在文件序列化/反序列化中特别有用，因为结构体通常可以被逐字写入文件。(例如，获取一个指向结构的指针，使用SIZE宏计算要复制的字节数，然后将数据移进或移出结构。)

类是一种不同类型的抽象数据类型，试图确保信息隐藏。在内部，可以有各种各样的诡计，方法，临时变量，状态变量。等等，它们都被用来为任何希望使用该类的代码提供一致的API。

实际上，结构体是关于数据的，类是关于代码的。

但是，您确实需要理解这些仅仅是抽象。创建看起来很像类的结构体和看起来很像结构体的类是完全可能的。事实上，最早的c++编译器只是将c++代码转换为C的预编译器。因此，这些抽象有助于逻辑思维，而不一定是计算机本身的资产。

Beyond the fact that each is a different type of abstraction, Classes provide solutions to the C code naming puzzle. Since you can't have more than one function exposed with the same name, developers used to follow a pattern of _(). e.g. mathlibextreme_max(). By grouping APIs into classes, similar functions (here we call them "methods") can be grouped together and protected from the naming of methods in other classes. This allows the programmer to organize his code better and increase code reuse. In theory, at least.

引用c++ FAQ，

[7.8] What's the difference between the keywords struct and class? The members and base classes of a struct are public by default, while in class, they default to private. Note: you should make your base classes explicitly public, private, or protected, rather than relying on the defaults. Struct and class are otherwise functionally equivalent. OK, enough of that squeaky clean techno talk. Emotionally, most developers make a strong distinction between a class and a struct. A struct simply feels like an open pile of bits with very little in the way of encapsulation or functionality. A class feels like a living and responsible member of society with intelligent services, a strong encapsulation barrier, and a well defined interface. Since that's the connotation most people already have, you should probably use the struct keyword if you have a class that has very few methods and has public data (such things do exist in well designed systems!), but otherwise you should probably use the class keyword.

这只是一种惯例。可以创建结构来保存简单的数据，但稍后会随着成员函数和构造函数的添加而发展。另一方面，在struct中看到public: access之外的东西是不寻常的。

唯一的其他区别是类和结构的默认继承，不出意外，它们分别是私有和公共的。