笔试之语言特性代码记录

#include <stdio.h>#include <sys/types.h>#include <unistd.h>int main(){    int ret = 0;    int i = 0;    for( i = 0; i < 2; i++ )    {        fork();        printf( "a\n" );    }    return ret;}// 输出6个a

#include <stdio.h>#include <iostream>using namespace std;class A{    public:        int &b;  //有这个sizeof(A)为16，在64位机器上，没有该引用，则为4        int a;};int main(){    int ret = 0;    cout << sizeof(int*) << " " << sizeof(A) << " " << sizeof(int) << endl; // 8 16 4 //  unsigned int a = 1;//  printf( "%d\n", a * (-2) );//  cout << a * -2 << endl;    unsigned int a = 0x1FFFFFF7;    unsigned char b = a;    char c = a;    char *p = ( char * )&a;    long long data = 0xf1f2f3f4f8f0f5f7;    cout << sizeof(long long) << endl;    printf( "%x, %x, %x, %x\n", b, c, *p, data );    long long a1 = 1, b1 = 2, c1 = 3;    printf( "%d, %d, %d\n", a1, b1, c1 );    return ret;}/*long long data = 0xf1fffffffffff5f7;8f7, fffffff7, fffffff7, fffff5f7%x总是输出4个字节，所以当不足4个字节时，如b，c，*p会补，对应无符号位补0，有符号位补1,对于超过4个字节的，也是将4个字节入栈；*/

1. #define定义的宏只在本文件有效 //myheader.h#ifndef MYFILE#define MYFILEvoid test();#endif// use1.cpp#include <iostream>using namespace std;//#include "myheader.h"#define MAX 10void func(){    #ifdef MYFILE        cout << "MYFILE is defined" << endl;      #else        cout << "MYFILE is not defined" << endl; //虽然在use1.cpp中包含了#include "myheader.h"，但是该文件中没有，故MYFILE并没有定义    #endif      return;}// use2.cpp#include "myheader.h"#include <iostream>using namespace std;extern void func();int main(){    func();    //cout << MAX << endl;   //会出现：error: ‘MAX’ was not declared in this scope    return 0;}g++ use1.cpp use2.cpp -o use./useMYFILE is not defined故下面代码是为了防止同一个文件包含一个头文件多次#ifndef MYFILE#define MYFILE#endif===========================#include <iostream>using namespace std;int cnt = 0;int show(){    cnt = 9;    return 0;}int main(){    cout << show() << ', ' << cnt << endl;  // 输出 0, 0    return 0;}

#include "myheader.h"#include <iostream>using namespace std;extern void func();extern const int v1;extern const int v2;class Test{    private:        static int a;};int Test::a = 10;// static int Test::a = 10; //编译出错void f( const int &a ) //可以编译通过，但若去掉const则编译出错{    cout << "test f(const int&)" << endl;    return;}//int mal( int a, int b = 2, int c = 3 );int mal( int a, int b = 2, int c = 4 ){    //int test()    //{     //  return 0;    //}    cout << a + b + c << endl;    return 0;}template <typename T> T add( T x, T y ){    return x + y;}class A{    public:        int a;        int &b;};// 分解质因数，任何数都可以用若干个质数相乘得到void prim( int m, int n ){    if( m >= n )    {        while( m % n != 0 )        {            ++n;        }        cout << n << endl;        m /= n;        prim( m, n );    }    return;}typedef int* PINT;typedef int *PINT1;// PINT等价于PINT1typedef int fun( int *, int );int testfun( int *p, int a ){    return a;}class ConstRef{    public:        ConstRef( int ii ) : i(ii), ci(i), ri(ii){}        void show()        {            cout << ci << ' ' << ri << endl;            ri = 8;            cout << ci << ' ' << ri << endl;        }    private:        int i;        const int ci;        int &ri;};int main(){    int hah = 0;    int &p33 = hah;    cout << sizeof(int) << ' ' << sizeof(const int) << ' ' << sizeof(p33) << endl;    cout << sizeof(ConstRef) << endl;    ConstRef obj(3);    obj.show();    int t1 = 9;    const PINT p = &t1;    const int *p2 = &t1;    const PINT1 p3 = &t1;    *p3 = 3;    *p = 10; //编译通过    //*p2 = 20;  //编译出错    //p = &t1;    //p3 = &t1; //编译出错 assignment of read-only variable ‘p3’    cout << t1 << endl;    int *p1;    //*p1 = 9;    int data = 27;    while( cin >> data )    {        prim( data, 2 );        cout << "===============================" << endl;    }    cout << "sizeof(A)=" << sizeof(A) << endl;    cout << "sizeof(int) = " << sizeof(int) << endl;    //add( 4.3, 2 );    add<float>( 4.3, 3 );    func();    //cout << MAX << endl;      //cout << v1 << endl; //访问出错    cout << v2 << endl;    const char *pp;    //(**pp) = 'c';    cout << mal( 2 ) << endl;    f(1);     return 0;}

#include <iostream>using namespace std;//#include "myheader.h"#define MAX 10const int v1 = 10; //c++中只能在当前文件中使用extern const int v2 = 20; //c++可以在其他文件中使用// c中const变量为外部链接，而c++中为内部链接void func(){    #ifdef MYFILE        cout << "MYFILE is defined" << endl;    #else        cout << "MYFILE is not defined" << endl;    #endif      return;}