某公司2014校园招聘笔试题C++

算是复习一下C++的东西。

1. What is the output of the program below in a Win32 platform? (10 points)

void size_func(char a[])
{
    printf("%d\n", sizeof(a));
}

int _tmain(int argc, _TCHAR* argv[])
{
    char str[] = "WelcomeToBJ";
    char *p = str;
    void *m = malloc(100);
    int n = 10;
    
    printf("%d\n", sizeof(str));
    printf("%d\n", sizeof(p));
    printf("%d\n", sizeof(n));
    printf("%d\n", sizeof(m));
    size_func(str);
    
    return 0;
}

The output is:

12

4

4

4

4

2. What are the outputs of Test() functions? Explain the reason for your answer. (10 points)

1.

void GetMemory(char *p)
{
    p = (char *)malloc(100);
}

void Test(void)
{
    char *str = NULL;
    GetMemory(str);
    strcpy(str, "Hello Beijing");
    printf(str);
}

2.

char *GetMemory(void)
{
    char p[] = "Hello Beijing";
    return p;
}

void Test(void)
{
    char *str = NULL;
    str = GetMemory();
    printf(str);
}

3.

void GetMemory3(char **p, int num)
{
    *p = (char *) malloc(num);
}

void Test3()
{

    char *str = NULL;
    GetMemory3(&str, 100);
    strcpy(str, "Hello Beijing");
    printf(str);
}

4.

void Test4()
{
    char *str = (char *)malloc(100);
    strcpy(str, "Hello");
    free(str);
    if(str != NULL)
    {
        strcpy(str, "Beijing");
        printf(str);
    }
}

Answer：1.显然不对，GetMemory函数传参的时候只是将str的值（也就是一个地址）赋给了p，在函数里给p赋值不影响外面的str值，所以在strcpy()的时候会出错。

2. 函数中的p指向的字符串应为local variable，随着函数调用结束而释放。

（如果不是定义的字符串“char p[] = "Hello Beijing";”， 而是定义的字符指针 char *p，那么可以正常执行。

因为初始化的字符串数组存储在栈中，可以改变其值。初始化的字符串常量指针存储在静态常量区，不能修改其值。）

3. 可以正常输出"Hello Beijing"

4. 输出为："Beijing"，在内存被free之后str并不等于NULL，但是str也是野指针，并不安全，可能会覆盖其他数据，所以一般会将指针free后直接将指针赋为空。

3. What is the output of the program below? (10 points)

class IGT
{
public:
    void x()    {cout << "class IGT, function x"<< endl;}
    virtual void y()    {cout << "class IGT, function y" << endl;}
};

class ADV : public IGT
{
public:
    virtual void x()    { cout << "class ADV, function x" << endl;}
    void y()    { cout << "class ADV, function y" << endl;}
};

class CTA : public ADV
{
public:
    void x()    {cout << "class CTA, function x" << endl;}
    void y()    {cout << "class CTA, function y" << endl;}
};

int tmain2()
{
    IGT* igt1 = new ADV;
    IGT* igt2 = new CTA;
    ADV* adv = new CTA;

    igt1->x();
    igt1->y();
    igt2->x();
    igt2->y();
    adv->x();
    adv->y();
    
    delete igt1;
    igt1 = NULL;

    delete igt2;
    igt2 = NULL;

    delete adv;
    adv = NULL;

    return 0;
}

The output :

class IGT, function x
class ADV, function y
class IGT, function x
class CTA, function y
class CTA, function x
class CTA, function y