sizeof运算符编译时或运行时

Question

我对sizeof操作员的评估时间感到困惑.
什么时候对sizeof运算符进行求值？

它的评估时间[ compile-time or run-time] 是否取决于语言[ C? C++?]？

我们可以sizeof在运行时[ in C++] 创建的对象中使用吗？

Answer 1

几乎在所有情况下,sizeof都是基于静态类型信息进行评估的(基本上是在编译时).

一个例外(我认为唯一的例子)就是C99的可变长度数组(VLA).

Answer 2

几乎总是编译时间.但您可能会对以下示例感兴趣:

char c[100];
sizeof(c); // 100

char* d = malloc(100);
sizeof(d); //probably 4 or 8.  tells you the size of the pointer!

BaseClass* b = new DerivedClass();
sizeof(b); //probably 4 or 8 as above.

void foo(char[100] x) {
    sizeof(x); //probably 4 or 8.  I hate this.  Don't use this style for this reason.
}

struct Foo {
    char a[100];
    char b[200];
};

sizeof(struct Foo); //probably 300.  Technically architecture dependent but it will be
//the # of bytes the compiler needs to make a Foo.

struct Foo foo;
sizeof(foo); //same as sizeof(struct Foo)

struct Foo* fooP;
sizeof(fooP); //probably 4 or 8

class ForwardDeclaredClass;

ForwardDeclaredClass* p;
sizeof(p); //4 or 8
ForwardDeclaredClass fdc; //compile time error.  Compiler
//doesn't know how many bytes to allocate

sizeof(ForwardDeclaredClass); //compile time error, same reason