帮我看看这个问题

[ratzip]

大家好，我在面试的时候被问到这样一个问题，

Test *t1 = new Test();
Test *t2 = new Test;

这个Test类里有一个用户自己定义的默认构造函数，那么上面两行代码都做了什么，有什么区别？

我知道这个new Test;是会去调用默认的构造函数， new Test()是调用没有参数的构造函数, 这个问题的题干没有Test类的定义，所以我也不知道用户自己定义的默认构造函数是什么样子，不知道有没有参数，那么怎么解答这个问题，默认的构造函数是什么样子的，怎么定义？

[老邓]

默认构造函数当然是没有参数的构造函数了。
class Test
{
Test() {} // 这就是默认的构造函数

所以两个用法没区别。
要说区别，那也只是多打了一个()而已。

[SoulRed]

还是有区别的。
有的时候。不加（）是无法实例化该类的。有些类没有默认的构造参数

[qiaolixian]

有区别
加（）是为了调用构造参数（无参数的）
不加（）有些类为了调用所有具有参数的构造参数

[xyq129052]

应该有区别 一个初始化一个没有吧

[luciferisnotsatan]

如果Test 类成员变量都是内建类型，同时没有自己写无参的构造函数，那么new Test不会初始化对象

class Test
{
public:
//	Test(){};
	int x;
};

Test *t1 = new Test();
00412A5E  mov         dword ptr [ebp-0F8h],4 
00412A68  mov         eax,dword ptr [ebp-0F8h] 
00412A6E  push        eax  
00412A6F  call        operator new (411622h) 
00412A74  add         esp,4 
00412A77  mov         dword ptr [ebp-0ECh],eax 
00412A7D  cmp         dword ptr [ebp-0ECh],0 
00412A84  je          main+6Ch (412AACh) 
00412A86  mov         ecx,dword ptr [ebp-0F8h] 
00412A8C  push        ecx  
00412A8D  push        0    
00412A8F  mov         edx,dword ptr [ebp-0ECh] 
00412A95  push        edx  
00412A96  call        @ILT+510(_memset) (411203h) 
00412A9B  add         esp,0Ch 
00412A9E  mov         eax,dword ptr [ebp-0ECh] 
00412AA4  mov         dword ptr [ebp-100h],eax 
00412AAA  jmp         main+76h (412AB6h) 
00412AAC  mov         dword ptr [ebp-100h],0 
00412AB6  mov         ecx,dword ptr [ebp-100h] 
00412ABC  mov         dword ptr [t1],ecx 
Test *t2 = new Test;
00412ABF  push        4    
00412AC1  call        operator new (411622h) 
00412AC6  add         esp,4 
00412AC9  mov         dword ptr [ebp-0E0h],eax 
00412ACF  mov         eax,dword ptr [ebp-0E0h] 
00412AD5  mov         dword ptr [t2],eax 

但如果显示写了，就没有区别。

class Test
{
public:
	Test(){};
	int x;
};

Test *t1 = new Test();
0041571D  push        4    
0041571F  call        operator new (411622h) 
00415724  add         esp,4 
00415727  mov         dword ptr [ebp-104h],eax 
0041572D  mov         dword ptr [ebp-4],0 
00415734  cmp         dword ptr [ebp-104h],0 
0041573B  je          main+70h (415750h) 
0041573D  mov         ecx,dword ptr [ebp-104h] 
00415743  call        Test::Test (41174Eh) 
00415748  mov         dword ptr [ebp-118h],eax 
0041574E  jmp         main+7Ah (41575Ah) 
00415750  mov         dword ptr [ebp-118h],0 
0041575A  mov         eax,dword ptr [ebp-118h] 
00415760  mov         dword ptr [ebp-110h],eax 
00415766  mov         dword ptr [ebp-4],0FFFFFFFFh 
0041576D  mov         ecx,dword ptr [ebp-110h] 
00415773  mov         dword ptr [ebp-14h],ecx 
Test *t2 = new Test;
00415776  push        4    
00415778  call        operator new (411622h) 
0041577D  add         esp,4 
00415780  mov         dword ptr [ebp-0ECh],eax 
00415786  mov         dword ptr [ebp-4],1 
0041578D  cmp         dword ptr [ebp-0ECh],0 
00415794  je          main+0C9h (4157A9h) 
00415796  mov         ecx,dword ptr [ebp-0ECh] 
0041579C  call        Test::Test (41174Eh) 
004157A1  mov         dword ptr [ebp-118h],eax 
004157A7  jmp         main+0D3h (4157B3h) 
004157A9  mov         dword ptr [ebp-118h],0 
004157B3  mov         eax,dword ptr [ebp-118h] 
004157B9  mov         dword ptr [ebp-0F8h],eax 
004157BF  mov         dword ptr [ebp-4],0FFFFFFFFh 
004157C6  mov         ecx,dword ptr [ebp-0F8h] 
004157CC  mov         dword ptr [ebp-20h],ecx 

如果成员有非内建类型，也没有区别

class Test
{
public:
	//Test(){};
	int x;
	string str;
};

Test *t1 = new Test();
00412A8D  push        24h  
00412A8F  call        operator new (411627h) 
00412A94  add         esp,4 
00412A97  mov         dword ptr [ebp-104h],eax 
00412A9D  mov         dword ptr [ebp-4],0 
00412AA4  cmp         dword ptr [ebp-104h],0 
00412AAB  je          main+70h (412AC0h) 
00412AAD  mov         ecx,dword ptr [ebp-104h] 
00412AB3  call        Test::Test (4115D7h) 
00412AB8  mov         dword ptr [ebp-118h],eax 
00412ABE  jmp         main+7Ah (412ACAh) 
00412AC0  mov         dword ptr [ebp-118h],0 
00412ACA  mov         eax,dword ptr [ebp-118h] 
00412AD0  mov         dword ptr [ebp-110h],eax 
00412AD6  mov         dword ptr [ebp-4],0FFFFFFFFh 
00412ADD  mov         ecx,dword ptr [ebp-110h] 
00412AE3  mov         dword ptr [ebp-14h],ecx 
Test *t2 = new Test;
00412AE6  push        24h  
00412AE8  call        operator new (411627h) 
00412AED  add         esp,4 
00412AF0  mov         dword ptr [ebp-0ECh],eax 
00412AF6  mov         dword ptr [ebp-4],1 
00412AFD  cmp         dword ptr [ebp-0ECh],0 
00412B04  je          main+0C9h (412B19h) 
00412B06  mov         ecx,dword ptr [ebp-0ECh] 
00412B0C  call        Test::Test (4115D7h) 
00412B11  mov         dword ptr [ebp-118h],eax 
00412B17  jmp         main+0D3h (412B23h) 
00412B19  mov         dword ptr [ebp-118h],0 
00412B23  mov         eax,dword ptr [ebp-118h] 
00412B29  mov         dword ptr [ebp-0F8h],eax 
00412B2F  mov         dword ptr [ebp-4],0FFFFFFFFh 
00412B36  mov         ecx,dword ptr [ebp-0F8h] 
00412B3C  mov         dword ptr [ebp-20h],ecx