潍坊外贸网站建设,泰州建设网站,怎么申请自媒体平台账号,帝国cms和wordpress哪个更好该楼层疑似违规已被系统折叠 隐藏此楼查看此楼http://ca21days.blog.163.com/blog/static/4383729420096102223744/变量的声明(declarations)会将变量的类型和名称传达给程序。当然#xff0c;定义(definitions)也是一种声明#xff1a;当我们定义一个变量的时候#xff0c;…该楼层疑似违规已被系统折叠 隐藏此楼查看此楼http://ca21days.blog.163.com/blog/static/4383729420096102223744/变量的声明(declarations)会将变量的类型和名称传达给程序。当然定义(definitions)也是一种声明当我们定义一个变量的时候我们当然也声明了他的名称和类型。我们可以通过使用“extern”关键字来声明(declarations)一个变量而不用定义(definitions)它。声明(declarations)的形式就是在对象(变量)的名字和类型前面加上关键字“extern”:extern int i; // declares but does not define iint i; // declares and defines iAn extern declaration is not a definitionand does not allocate storage. In effect, it claims that a definition ofthe variable exists elsewhere in the program. A variable can bedeclared multiple times in a program, but it must be defined only once.带“extern”关键字的语句属于声明(declarations)不是定义(definitions)他不会给变量分配内存。实际上它是说明程序中的某处包含这个变量的定义。一个变量可以在程序中被声明(declarations)多次但是只能被定义(definitions)一次。A declaration may have an initializer only if it is also a definition because onlya definition allocates storage. The initializer must have storage toinitialize. If an initializer is present, the declaration is treated as adefinition even if the declaration is labeled extern:声明(declarations)时你可以给变量初始化。但是一旦你这样做那么这句话也就变成了定义(definitions)因为只有在定义(definitions)的时候才会为变量分配内存。初始化的时候必然要为初始值分配存储空间。如果你在声明(declarations)的时候同时初始化了变量即便“extern”关键字存在这个语句也会认为是定义(definitions)。extern double pi 3.1416; // definitionDespite the use of extern, this statementdefines pi. Storage is allocated and initialized. An extern declarationmay include an initializer only if it appears outside a function.不管有没有“extern”关键字存在这条语句的作用也是定义(definitions)“pi”。变量已经被分配了内存并且赋予了初始值。声明(declarations)只有在一种情况下可以被初始化那就是当他被放置在函数外部的时候。
