c – 在控制结构块中定义变量

If I define a variable inside the block of a control structure,does it exist only in the execution of that control structure’s block and not in the whole execution of the enclosing function?

这取决于您声明变量的位置而不是定义它.

该变量只能在您声明它的范围内访问.如果您明确传递它,但是如果它保持有效则取决于变量的存储类型,可以在范围之外访问它.
例如：静态变量在整个程序生命周期内保持活动状态,同时,
从函数返回自动变量的地址将导致未定义的行为,因为该函数返回后变量不会保持有效.

好读：
What is the difference between a definition and a declaration?

How can I monitor the exact memory usage of my programs and its changes (i.e.: changes in memory usage by creating and destroying variables)?

我相信您会希望获得有关动态分配对象的信息,因为自动对象只能在其范围内存活足够长时间,它们将自动销毁,因此它们通常不会导致任何问题.

对于动态对象您可以使用内存分析工具(如valgrind with Massif)或replace new and delete operators for your class来收集诊断信息.

编辑：解决更新的问题.

In following code I know v scope is if block,but I want to know whether v is created/destroyed in the memory at the start/end of the if block or at the start/end of the function func?

当声明它的作用域开始并且声明它的语句被执行时,就会创建v.一旦达到范围,即销毁v.
这个概念用于形成C中最广泛使用的概念之一的基础,称为Resource Allocation is Initialization(RAII).每个C程序员绝对必须知道它.

通过这个小的修改过的code sample,演示和验证对象的创??建和销毁很简单：

#include<iostream>
class Myclass
{
    public:
        Myclass(){std::cout<<"nIn Myclass Constructor ";}
        ~Myclass(){std::cout<<"nIn Myclass Destructor";}
};

void func()
{
    std::cout<<"nBefore Scope Begins";
    if (true)
    {
        Myclass obj;//automatic storage class
    }
    std::cout<<"nAfter Scope Ends";
}

int main()
{
    std::cout<<"nBefore Calling func()";
    func();
    std::cout<<"nAfter Calling func()";
    return 0;
}

输出是：

Before Calling func() Before Scope Begins In Myclass Constructor In Myclass Destructor After Scope Ends After Calling func()