新的 c++11 for 循环导致：“错误：'begin' 未在此范围内声明”
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new c++11 for loop causes: "error: ‘begin’ was not declared in this scope"
提问by bercio
I'm trying to learn c++, so I wrote a short program that uses the new c++11 for loop, which makes the compiler give me an error I don't understand. this is my c++ code:
我正在尝试学习c++，所以我写了一个使用新的c++11 for循环的简短程序，这使得编译器给我一个我不明白的错误。这是我的 C++ 代码：
#include <iostream>
#include <cmath>
using namespace std;
float legge_oraria_moto_accelerato(float a[3]){
return a[2]*a[0] + 0.5*a[1]*a[0]*a[0];
}
int corri(float (f)(float array[3]), float arrays[3][3])
{ for(auto i:arrays) cout << f(i) << '\n';
return 0;
}
int main()
{
return 0;
}
and this is the compiler's (g++ -std=gnu++11) error:
这是编译器的 (g++ -std=gnu++11) 错误：
mezzo.cpp: In function ‘int corri(float (*)(float*), float (*)[3])':
mezzo.cpp:9:18: error: ‘begin' was not declared in this scope
{ for(auto i:arrays) cout << f(i) << '\n';
^
mezzo.cpp:9:18: note: suggested alternatives:
In file included from /usr/include/c++/4.9/bits/basic_string.h:42:0,
from /usr/include/c++/4.9/string:52,
from /usr/include/c++/4.9/bits/locale_classes.h:40,
from /usr/include/c++/4.9/bits/ios_base.h:41,
from /usr/include/c++/4.9/ios:42,
from /usr/include/c++/4.9/ostream:38,
from /usr/include/c++/4.9/iostream:39,
from mezzo.cpp:1:
/usr/include/c++/4.9/initializer_list:89:5: note: ‘std::begin'
begin(initializer_list<_Tp> __ils) noexcept
^
/usr/include/c++/4.9/initializer_list:89:5: note: ‘std::begin'
mezzo.cpp:9:18: error: ‘end' was not declared in this scope
{ for(auto i:arrays) cout << f(i) << '\n';
^
mezzo.cpp:9:18: note: suggested alternatives:
In file included from /usr/include/c++/4.9/bits/basic_string.h:42:0,
from /usr/include/c++/4.9/string:52,
from /usr/include/c++/4.9/bits/locale_classes.h:40,
from /usr/include/c++/4.9/bits/ios_base.h:41,
from /usr/include/c++/4.9/ios:42,
from /usr/include/c++/4.9/ostream:38,
from /usr/include/c++/4.9/iostream:39,
from mezzo.cpp:1:
/usr/include/c++/4.9/initializer_list:99:5: note: ‘std::end'
end(initializer_list<_Tp> __ils) noexcept
^
/usr/include/c++/4.9/initializer_list:99:5: note: ‘std::end'
回答by interjay
Range-based for loops work with arrays, but not with pointers. The issue here is that arraysis actually a pointer and not an array.
基于范围的 for 循环适用于数组，但不适用于指针。这里的问题是它arrays实际上是一个指针而不是一个数组。
When you have a function parameter that is declared as an array, it is adjusted to a pointer type. You can see this here with the parameter float arrays[3][3]: In the compiler error message you can see that the actual parameter type is a pointer to an array float (*)[3], which can't be used with a ranged-based for loop.
当您有一个声明为数组的函数参数时，它会被调整为指针类型。您可以在此处通过参数看到这一点float arrays[3][3]：在编译器错误消息中，您可以看到实际参数类型是指向数组的指针float (*)[3]，它不能与基于范围的 for 循环一起使用。
If you pass the array by reference instead (float (&arrays)[3][3]), it won't adjusted to a pointer in this manner and will therefore work with the range-based for loop.
如果您改为通过引用传递数组 ( float (&arrays)[3][3])，它将不会以这种方式调整为指针，因此将与基于范围的 for 循环一起使用。
