我试图了解隐式类成员初始化如何对成员{POD结构,POD类和POD}起作用。在阅读了一下后,我预计它们会被初始化为默认值,但实际行为似乎有所不同 -
#include <iostream>
struct S1
{
void* a;
int b;
};
struct S2
{
S2() { std::cout << "!"; }
void* a;
int b;
};
struct S3
{
S3() : a(), b() { std::cout << "!"; }
void* a;
int b;
};
class C1
{
public:
void* a;
int b;
};
class C2
{
public:
C2() { std::cout << "!"; }
void* a;
int b;
};
class C3
{
public:
C3() : a(), b() { std::cout << "!"; }
void* a;
int b;
};
template <typename T>
class FOO1
{
public:
T s;
int a;
};
template <typename T>
class FOO2
{
public:
FOO2() {}
T s;
int a;
};
template <typename T>
class FOO3
{
public:
FOO3() : s(), a() {}
T s;
int a;
};
//#define SKIP_S1C1
template <typename T>
void moo()
{
#ifndef SKIP_S1C1
T* f = new T();
T foo = *f;
std::cout << ":\ts = (" << foo.s.a << ", " << foo.s.b << ")\ta = " << foo.a << std::endl;
delete f;
#else
T foo;
std::cout << ":\ts = (" << foo.s.a << ", " << foo.s.b << ")\ta = " << foo.a << std::endl;
#endif
}
int main()
{
#ifndef SKIP_S1C1
moo<FOO1<S1> >();
#endif
moo<FOO1<S2> >();
moo<FOO1<S3> >();
#ifndef SKIP_S1C1
moo<FOO1<C1> >();
#endif
moo<FOO1<C2> >();
moo<FOO1<C3> >();
std::cout << std::endl;
#ifndef SKIP_S1C1
moo<FOO2<S1> >();
#endif
moo<FOO2<S2> >();
moo<FOO2<S3> >();
#ifndef SKIP_S1C1
moo<FOO2<C1> >();
#endif
moo<FOO2<C2> >();
moo<FOO2<C3> >();
std::cout << std::endl;
#ifndef SKIP_S1C1
moo<FOO3<S1> >();
#endif
moo<FOO3<S2> >();
moo<FOO3<S3> >();
#ifndef SKIP_S1C1
moo<FOO3<C1> >();
#endif
moo<FOO3<C2> >();
moo<FOO3<C3> >();
}
明显的运行结果不足以说明POD是初始化为默认值0还是仅包含噪声。但无论如何,这里有一些结果:
在ubuntu上使用gcc 4.6.3构建并运行它#define SKIP_S1C1
取消注释,我得
!: s = (0x7ffffe557770, 4196620) a = 1
!: s = (0, 0) a = 1
!: s = (0, 0) a = 1
!: s = (0, 0) a = 1
!: s = (0x1, 6299744) a = 6299744
!: s = (0, 0) a = 6299744
!: s = (0, 0) a = 6299744
!: s = (0, 0) a = 6299744
!: s = (0x1, 6299744) a = 0
!: s = (0, 0) a = 0
!: s = (0, 0) a = 0
!: s = (0, 0) a = 0
随着它的注释,我得到了
: s = (0, 0) a = 0
!: s = (0, 0) a = 0
!: s = (0, 0) a = 0
: s = (0, 0) a = 0
!: s = (0, 0) a = 0
!: s = (0, 0) a = 0
: s = (0, 0) a = 0
!: s = (0, 0) a = 0
!: s = (0, 0) a = 0
: s = (0, 0) a = 0
!: s = (0, 0) a = 0
!: s = (0, 0) a = 0
: s = (0, 0) a = 0
!: s = (0, 0) a = 0
!: s = (0, 0) a = 0
: s = (0, 0) a = 0
!: s = (0, 0) a = 0
!: s = (0, 0) a = 0
和VS2013一起评论,
: s = (00000000, 0) a = 0
!: s = (CDCDCDCD, -842150451) a = -842150451
!: s = (00000000, 0) a = -842150451
: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
: s = (CDCDCDCD, -842150451) a = -842150451
!: s = (CDCDCDCD, -842150451) a = -842150451
!: s = (00000000, 0) a = -842150451
: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
: s = (00000000, 0) a = 0
!: s = (CDCDCDCD, -842150451) a = 0
!: s = (00000000, 0) a = 0
: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
并取消注释,
!: s = (CCCCCCCC, -858993460) a = -858993460
!: s = (00000000, 0) a = -858993460
!: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
!: s = (CCCCCCCC, -858993460) a = -858993460
!: s = (00000000, 0) a = -858993460
!: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
!: s = (CCCCCCCC, -858993460) a = 0
!: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
!: s = (00000000, 0) a = 0
我真的很想了解{POS struct,POD classes和POD}成员的隐式初始化时我应该期待什么以及何时是UB。任何帮助将不胜感激...... :)
答案 0 :(得分:4)
构造函数很复杂,细节是技术性的,但这里是一个通用摘要*:
初始化有三种方法:
他们在很多情况下被召唤:
new T;
- 默认初始化new T();
- 值初始化有关更多详细信息,请参阅C ++ 11草案中的§8.5和§12.6部分。他们很长很乏味。
另请注意,C的规则在技术上有惊人的不同,尽管效果对我来说是一样的。
*我的摘要在技术上并不准确,但对于大多数实际代码而言足够准确。例如,数组在技术上有特殊的规则,但它们非常直观,不值得一提。
**是的,它的“初始化”是“无初始化”,这使得其他段落“如果它被初始化”在技术上含糊不清,但应用常识。它没有初始化。