::是运算符中等级最高的,它分为三种:
1)global scope(全局作用域符),用法(::name)
2)class scope(类作用域符),用法(class::name)
3)namespace scope(命名空间作用域符),用法(namespace::name)
他们都是左关联(left-associativity)
他们的作用都是为了更明确的调用你想要的变量,如在程序中的某一处你想调用全局变量a,那么就写成::a,如果想调用class A中的成员变量a,那么就写成A::a,另外一个如果想调用namespace std中的cout成员,你就写成std::cout(相当于using namespace
std;cout)意思是在这里我想用cout对象是命名空间std中的cout(即就是标准库里边的cout)
比如几天在看一个TCP的网络程序时,程序中自己定义了一个类封装了IP链接,而且名称也是用的connect,具体如下
class IPStack { public: ........ ........ int connect(const char* hostname, int port) { int type = SOCK_STREAM; struct sockaddr_in address; int rc = -1; sa_family_t family = AF_INET; struct addrinfo *result = NULL; struct addrinfo hints = {0, AF_UNSPEC, SOCK_STREAM, IPPROTO_TCP, 0, NULL, NULL, NULL}; if ((rc = getaddrinfo(hostname, NULL, &hints, &result)) == 0) { struct addrinfo* res = result; /* prefer ip4 addresses */ while (res) { if (res->ai_family == AF_INET) { result = res; break; } res = res->ai_next; } if (result->ai_family == AF_INET) { address.sin_port = htons(port); address.sin_family = family = AF_INET; address.sin_addr = ((struct sockaddr_in*)(result->ai_addr))->sin_addr; } else rc = -1; freeaddrinfo(result); } if (rc == 0) { mysock = socket(family, type, 0); if (mysock != -1) { int opt = 1; //if (setsockopt(mysock, SOL_SOCKET, SO_NOSIGPIPE, (void*)&opt, sizeof(opt)) != 0) // printf("Could not set SO_NOSIGPIPE for socket %d", mysock); rc = ::connect(mysock, (struct sockaddr*)&address, sizeof(address)); //调用全局的connect函数即socket.h里面的connect函数 } } return rc; }
::的用法算是C++中的基础,如果运用的好的话,你程序的出错率也许会降很多