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分享const引用会延长临时变量的寿命与自己一样？
如果一个函数返回一个临时变量，const引用绑定这个临时变量，那么它会延长这个临时变量的寿命与自己一样？
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lx111000lx0 2016-03-22
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楼主如果是个菇凉，我可以告诉你去看看
http://itlab.idcquan.com/c/vc/200906/785943.html
写的很详细，左值和右值的问题。
shenxiaolong1976 2014-01-06
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为什么去掉const，G++报告错误？[/quote]
这是G++编译器的问题，不是C++语法的问题。关于const，其涉及到internal linkage/external linkage问题，和楼主所说的问题无关。
我专门测试了一下，在MS编译器上OK。
FeelTouch Labs 2013-08-06
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为什么去掉const，G++报告错误？[/quote]
虽然没有看到你的代码，大致感觉到，去掉const后一定是更改了某个不可更改的操作
参考：http://blog.chinaunix.net/uid-26748613-id-3378023.html
rocktyt 2013-08-06
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标准有明确规定声明周期要延长到和引用的一样
5 The second context is when a reference is bound to a temporary. The temporary to which the reference is
bound or the temporary that is the complete object of a subobject to which the reference is bound persists
for the lifetime of the reference except:
— A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the
constructor exits.
— A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of
the full-expression containing the call.
— The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is not
extended; the temporary is destroyed at the end of the full-expression in the return statement.
— A temporary bound to a reference in a new-initializer (5.3.4) persists until the completion of the
full-expression containing the new-initializer.
A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:
— If the reference is an lvalue reference and the initializer expression
（略）
— Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., cv1 shall be
const), or the reference shall be an rvalue reference.
rocktyt 2013-08-06
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标准有明确规定声明周期要延长到和引用的一样
5 The second context is when a reference is bound to a temporary. The temporary to which the reference is
bound or the temporary that is the complete object of a subobject to which the reference is bound persists
for the lifetime of the reference except:
— A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the
constructor exits.
— A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of
the full-expression containing the call.
— The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is not
extended; the temporary is destroyed at the end of the full-expression in the return statement.
— A temporary bound to a reference in a new-initializer (5.3.4) persists until the completion of the
full-expression containing the new-initializer.
A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:
— If the reference is an lvalue reference and the initializer expression
（略）
— Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., cv1 shall be
const), or the reference shall be an rvalue reference.
看到机器就疯狂 2013-08-06
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标准有明确规定声明周期要延长到和引用的一样
5 The second context is when a reference is bound to a temporary. The temporary to which the reference is
bound or the temporary that is the complete object of a subobject to which the reference is bound persists
for the lifetime of the reference except:
— A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the
constructor exits.
— A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of
the full-expression containing the call.
— The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is not
extended; the temporary is destroyed at the end of the full-expression in the return statement.
— A temporary bound to a reference in a new-initializer (5.3.4) persists until the completion of the
full-expression containing the new-initializer.
A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:
— If the reference is an lvalue reference and the initializer expression
（略）
— Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., cv1 shall be
const), or the reference shall be an rvalue reference.
rocktyt 2013-08-06
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标准有明确规定声明周期要延长到和引用的一样
5 The second context is when a reference is bound to a temporary. The temporary to which the reference is
bound or the temporary that is the complete object of a subobject to which the reference is bound persists
for the lifetime of the reference except:
— A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the
constructor exits.
— A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of
the full-expression containing the call.
— The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is not
extended; the temporary is destroyed at the end of the full-expression in the return statement.
— A temporary bound to a reference in a new-initializer (5.3.4) persists until the completion of the
full-expression containing the new-initializer.
A reference to type “cv1 T1” is initialized by an expression of type “cv2 T2” as follows:
— If the reference is an lvalue reference and the initializer expression
（略）
— Otherwise, the reference shall be an lvalue reference to a non-volatile const type (i.e., cv1 shall be
const), or the reference shall be an rvalue reference.
看到机器就疯狂 2013-08-06
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标准有明确规定声明周期要延长到和引用的一样
5 The second context is when a reference is bound to a temporary. The temporary to which the reference is
bound or the temporary that is the complete object of a subobject to which the reference is bound persists
for the lifetime of the reference except:
— A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the
constructor exits.
— A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of
the full-expression containing the call.
— The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is not
extended; the temporary is destroyed at the end of the full-expression in the return statement.
— A temporary bound to a reference in a new-initializer (5.3.4) persists until the completion of the
full-expression containing the new-initializer.mujiok2003 2013-08-06
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标准有明确规定声明周期要延长到和引用的一样
5 The second context is when a reference is bound to a temporary. The temporary to which the reference is
bound or the temporary that is the complete object of a subobject to which the reference is bound persists
for the lifetime of the reference except:
— A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the
constructor exits.
— A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of
the full-expression containing the call.
— The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is not
extended; the temporary is destroyed at the end of the full-expression in the return statement.
— A temporary bound to a reference in a new-initializer (5.3.4) persists until the completion of the
full-expression containing the new-initializer.mujiok2003 2013-08-06
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#include <string>
std::string foo(){ return "bar";}
int main(int argc,char* argv[])
{
//case one
{
std::string const& f = foo();
}
//case two
{
foo();
}
}
//case one
{
std::string const& f = foo();
010B44B8 lea eax,[ebp-30h]
010B44BB push eax
010B44BC call foo (010B11EFh)
010B44C1 add esp,4
010B44C4 lea ecx,[ebp-30h]
010B44C7 mov dword ptr [ebp-0Ch],ecx
}
010B44CA lea ecx,[ebp-30h]
010B44CD call std::basic_string<char,std::char_traits<char>,std::allocator<char> >::~basic_string<char,std::char_traits<char>,std::allocator<char> > (010B12B7h)
//延长到}
//case two
{
foo();
010B44D2 lea eax,[ebp-114h]
010B44D8 push eax
010B44D9 call foo (010B11EFh)
010B44DE add esp,4
010B44E1 lea ecx,[ebp-114h]
010B44E7 call std::basic_string<char,std::char_traits<char>,std::allocator<char> >::~basic_string<char,std::char_traits<char>,std::allocator<char> > (010B12B7h)
//立即析构
}
}
rocktyt 2013-08-06
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标准有明确规定声明周期要延长到和引用的一样
5 The second context is when a reference is bound to a temporary. The temporary to which the reference is
bound or the temporary that is the complete object of a subobject to which the reference is bound persists
for the lifetime of the reference except:
— A temporary bound to a reference member in a constructor’s ctor-initializer (12.6.2) persists until the
constructor exits.
— A temporary bound to a reference parameter in a function call (5.2.2) persists until the completion of
the full-expression containing the call.
— The lifetime of a temporary bound to the returned value in a function return statement (6.6.3) is not
extended; the temporary is destroyed at the end of the full-expression in the return statement.
— A temporary bound to a reference in a new-initializer (5.3.4) persists until the completion of the
full-expression containing the new-initializer.mujiok2003 2013-08-06
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取决于实现,没有强制要求。 VC好像会延长生命。
rocktyt 2013-08-06
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为什么去掉const，G++报告错误？[/quote]规定就是这样，右值不能被绑定到左值引用，但可以绑定到常量左值引用和右值引用
rocktyt 2013-08-06
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一般来说是的，但是要注意当const引用绑定到值返回的函数返回值时，生命周期没有被延长，函数调用那行语句一结束对象就会被释放，对引用的操作将会是不可预料的
看到机器就疯狂 2013-08-06
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为什么去掉const，G++报告错误？
max_min_ 2013-08-06
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const 修饰表示只读数据段， 与生命周期无关的
只读数据段仅是实现的其中一种方式，目的是在运行时令试图修改const变量的行为产生错误。
由于C标准并没有禁止对const变量的修改，而是规定属于未定义行为，
因此一个实现对于试图修改const的行为如何处理都没有违反标准，也就是说，
无论把const变量放在只读段也好，放在可被修改的地方也好（例如你说的栈），都是允许的行为。
starytx 2013-08-06
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返回的是临时变量而不是临时变量的地址，所以会复制生成一个临时对象，引用绑定的实际是那个临时对象，和函数内的临时变量没有关系了
