而在c++ 中经常使用的 using namespace std 语句就是第一种。 std 即为 c++ 中标准库中的标识符所在的命名空间的名字。 参考: Incomputing, anamespaceis a set of signs (names) that are used to identify and refer to objects of various kinds. A namespace ensures that all of a given set of ...
任何情况下都不要using namespace std从理论上来说也是有道理的：因为系统库可能会升级，这样升级编译使...
using 声明的形式如下: usingnamespace::name; 一旦使用了 using 声明,我们就可以直接引用名字,而不需要再引用该名字的命名空间。 #include <string>#include<iostream>//using declarations states our intent to use these names from thenamespacestdusingstd::cin;usingstd::string;intmain() {strings;//ok: ...
namespace std using std::cin;using std::string;int main(){ string s; // ok: string is now a synonym for std::string cin >> s; // ok: cin is now a synonym for std::cin cout << s; // error: no using declaration; we must use full name std::cout << s; // ok: ...
usingnamespace::name; 1. 一旦使用了 using 声明,我们就可以直接引用名字,而不需要再引用该名字的命名空间。 #include <string>#include<iostream>//using declarations states our intent to use these names from thenamespacestdusingstd::cin;usingstd::string;intmain() ...
1error C2027:use of undefined type 'A' see declaration of 'A#includeusing namespace std;class A;class B{public :B(A * a){a->c = 5;}};class A{public:int c;void set(){B b(this);cout 2error C2027: use of undefined type 'A' see declaration of 'A#includeusing namespace st...
1"unsafe use of type 'bool' in operation" 我的程序是#includeusing namespace std;void main(){char c1='a',c2='b',c3='c';int i1=10,i2=20,i3=30;double d1=0.1,d2=0.2,d3=0.3;double x;x=i1>i2>i3 2i2>i3">"unsafe use of type 'bool' in operation" 我的程序是#includeusin...
An exception is using namespace std::literals;. This is necessary to use string literals in header files and given the rules - users are required to name their own UDLs operator""_x - they will not collide with the standard library. ...
namespace NS_A1 { namespace NS_A2 { namespace NS_A3 { namespace { using NS_B1::NS_B2::NS_B3::ClassB; // some local variables } class A { // ... use ClassB methods } } } } unnamed namespace scope unnamed namespace不同于常规的namespace之处 首先想搞明白的就是unnamed namesp...
An exception is using namespace std::literals;. This is necessary to use string literals in header files and given the rules - users are required to name their own UDLs operator""_x - they will not collide with the standard library. ...