c++ - 编译错误 : loop through each element of an array inside a c++ function

Question

我是 C++ 的新手，我试图在 C++ 函数中遍历数组的每个元素，但出现编译时错误。有人可以解释为什么会这样吗？

这是我的代码:

#include <iostream>

int getMax(int arr[])
{
    int max = arr[0];
    for (int x: arr){
        if ( x > max ) max = x;
    }
    return max;
}


int main()
{

    int arr[] = {4,2,5,3,6,8,1};
    int max = getMax(arr);

    std::cout << max << std::endl;

    return 0;
}

---

这是错误信息:

get_max.cpp: In function 'int getMax(int*)':
get_max.cpp:6:17: error: 'begin' was not declared in this scope
     for (int x: arr){
                 ^
get_max.cpp:6:17: note: suggested alternative:
In file included from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/string:51:0,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/bits/locale_classes.h:40,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/bits/ios_base.h:41,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/ios:42,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/ostream:38,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/iostream:39,
                 from get_max.cpp:1:
C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/bits/range_access.h:87:5: note:   'std::begin'
     begin(_Tp (&__arr)[_Nm])
     ^
get_max.cpp:6:17: error: 'end' was not declared in this scope
     for (int x: arr){
                 ^
get_max.cpp:6:17: note: suggested alternative:
In file included from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/string:51:0,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/bits/locale_classes.h:40,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/bits/ios_base.h:41,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/ios:42,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/ostream:38,
                 from C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/iostream:39,
                 from get_max.cpp:1:
C:/Program Files (x86)/CodeBlocks/MinGW/lib/gcc/mingw32/5.1.0/include/c++/bits/range_access.h:97:5: note:   'std::end'
     end(_Tp (&__arr)[_Nm])

Answer 1

您必须通过引用传递数组以保留大小信息:

template <std::size_t N>
int getMax(const int (&arr)[N])
{
    int max = arr[0];
    for (int x: arr){
        if ( x > max ) max = x;
    }
    return max;
}

否则您必须使用指针传递大小。

int getMax(const int *arr, std::size_t size)
{
    assert(arr != nullptr);
    assert(size != 0);
    return *std::max_element(arr, arr + size);
}

使用 std::vector/std::array 具有更自然的语法。

int getMax(const std::vector<int>& arr)
{
    assert(!arr.empty());
#if 1
    return *std::max_element(arr.begin(), arr.end());
#else
    int max = arr[0];
    for (int x: arr){
        if ( x > max ) max = x;
    }
    return max;
#endif
}
int main()
{
    const std::vector<int> arr{4,2,5,3,6,8,1};
    const int max = getMax(arr);

    std::cout << max << std::endl;
}