C++中内置变量的初始化 - HelloWorld开发者社区

对于全局的变量

如果内置类型的变量未被显示地初始化，它的值将由定义的位置决定。

(1).定义在函数体之外的变量将被初始化为0；

(2).定义在函数体内部的变量将不被初始化，它的值将是任意的。

对于(1)举例如下：

short sn;
int in;
long ln;
long long lln;
float f;
double d;
long double ld;
bool b;
char c;
wchar_t wc;
int nArr[10];

void PrintVariable() 
{
    cout << "short:" << sn << endl;
    cout << "int:" << in << endl;
    cout << "long:" << ln << endl;
    cout << "long long:" << lln << endl;
    cout << "float:" << f << endl;
    cout << "double:" << d << endl;
    cout << "long double:" << ld << endl;
    cout << "bool:" << "he" << c << "llo" << endl;
    cout << "char:" << c << endl;
    cout << "wchar_t：" << "he" << wc << "llo" << endl;
    cout << "print array:" << endl;
    for (int i = 0; i < 10; i ++)
    {
        cout << nArr[i] << "    ";
    }
    cout << endl;
}

结果

short:0
int:0
long:0
long long:0
float:0
double:0
long double:0
bool:he llo
char:
wchar_t:he0llo
print array:
0 0 0 0 0 0 0 0 0 0

对于(1)举例如下：

void PrintVariable() 
{
    short sn;
    int in;
    int nArr[10];

    int num = in;                    //未被初始化，不请允许拷贝：Run-Time Check Failure #3 - The variable 'in' is being used without being initialized.
    cout << "short:" << sn << endl;            //未被初始化,不允许访问该成员：Run-Time Check Failure #3 - The variable 'sn' is being used without being initialized.
    cout << nArr[0] << in << endl;            //未被初始化,不允许访问该成员：Run-Time Check Failure #3 - The variable 'in' is being used without being initialized.
}

类内的成员变量

如果是在类中定义的类成员，则初始化的顺序为：

1.构造函数初始化

2.如果没有构造函数，则通过类内的初始值进行初始化(可能有些较老的版本不允许有类内初始值)

3.默认初始化(值将是未定义的，是任意的)

没有构造函数初始化：

#pragma once

#include <iostream>
using namespace std;

class TestData
{

public:
    //TestData(void);
    ~TestData(void);

    void PrintVariable() 
    {
        cout << "short:" << sn << endl;
        cout << "int:" << in << endl;
        cout << "long:" << ln << endl;
        cout << "long long:" << lln << endl;
        cout << "float:" << f << endl;
        cout << "double:" << d << endl;
        cout << "long double:" << ld << endl;
        cout << "bool:" << "he" << c << "llo" << endl;
        cout << "char:" << c << endl;
        cout << "wchar_t：" << "he" << wc << "llo" << endl;
        cout << "print array:" << endl;
        for (int i = 0; i < 10; i ++)
        {
            cout << nArr[i] << "    ";
        }
        cout << endl;
    }
private:
    short sn;
    int in;
    long ln;
    long long lln;
    float f;
    double d;
    long double ld;
    bool b;
    char c;
    wchar_t wc;
    int nArr[10];

};

结果(很可怕)：

short:-13108
int:-858993460
long:-858993460
long long:-3689348814741910324
float:-1.07374e+008
double:-9.25596e+061
long double:-9.25596e+061
bool:he蘬lo
char:
wchar_t：he52428llo
print array:
-858993460 -858993460 -858993460 -858993460 -858993460 -858993460
-858993460 -858993460 -858993460 -858993460

通过构造函数初始化

#pragma once

#include <iostream>
using namespace std;

class TestData
{

public:
    TestData(void) : sn(0), in(0), ln(0), lln(0), f(0), d(0.0), ld(0), b(true), c(' '), wc(L' ')
    {
        memset(nArr, 0, 10*sizeof(int));
    }

    ~TestData(void);

    void PrintVariable() 
    {
        cout << "short:" << sn << endl;
        cout << "int:" << in << endl;
        cout << "long:" << ln << endl;
        cout << "long long:" << lln << endl;
        cout << "float:" << f << endl;
        cout << "double:" << d << endl;
        cout << "long double:" << ld << endl;
        cout << "bool:" << "he" << c << "llo" << endl;
        cout << "char:" << c << endl;
        cout << "wchar_t：" << "he" << wc << "llo" << endl;
        cout << "print array:" << endl;
        for (int i = 0; i < 10; i ++)
        {
            cout << nArr[i] << "    ";
        }
        cout << endl;
    }
private:
    short sn;
    int in;
    long ln;
    long long lln;
    float f;
    double d;
    long double ld;
    bool b;
    char c;
    wchar_t wc;
    int nArr[10];

};

结果：

short:0
int:0
long:0
long long:0
float:0
double:0
long double:0
bool:he llo
char:
wchar_t：he32llo
print array:
0 0 0 0 0 0 0 0 0 0