为什么std :: set似乎强制使用const_iterator? [英] Why does std::set seem to force the use of a const_iterator?
问题描述
考虑下面的简单程序,该程序尝试使用对其中元素的NON-const引用来迭代集合的值:
Consider the simple program below, which attempts to iterate through the values of a set using NON-const references to the elements in it:
#include <set>
#include <iostream>
class Int
{
public:
Int(int value) : value_(value) {}
int value() const { return value_; }
bool operator<(const Int& other) const { return value_ < other.value(); }
private:
int value_;
};
int
main(int argc, char** argv) {
std::set<Int> ints;
ints.insert(10);
for (Int& i : ints) {
std::cout << i.value() << std::endl;
}
return 0;
}
编译时,我从gcc收到错误:
When compiling this, I get an error from gcc:
test.c: In function ‘int main(int, char**)’:
test.c:18:18: error: invalid initialization of reference of type ‘Int&’ from expression of type ‘const Int’
for (Int& i : ints) {
^
是的,我知道我实际上并不是在尝试修改for循环中的元素.但是关键是我应该能够在循环内使用非常量引用,因为集合本身不是常量限定的.如果创建一个setter函数并在循环中使用它,则会出现相同的错误.
Yes, I know I'm not actually trying to modify the elements in the for loop. But the point is that I should be able to get a non-const reference to use inside the loop, since the set itself is not const qualified. I get the same error if I create a setter function and use that in the loop.
推荐答案
集合就像地图一样,没有值,只有键.由于这些键用于加速集合上操作的树,因此它们无法更改.因此,所有元素必须为常量,以防止破坏基础树的约束.
A set is like a map with no values, only keys. Since those keys are used for a tree that accelerates operations on the set, they cannot change. Thus all elements must be const to keep the constraints of the underlying tree from being broken.
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