C++ Question
What's the difference between & and && in a range-based for loop?
I'm wondering what's the difference between
for (auto& i : v)for (auto&& i : v)#include <iostream>
#include <vector>
int main()
{
std::vector<int> v = {0, 1, 2, 3, 4, 5};
std::cout << "Initial values: ";
for (auto i : v) // Prints the initial values
std::cout << i << ' ';
std::cout << '\n';
for (auto i : v) // Doesn't modifies v because i is a copy of each value
std::cout << ++i << ' ';
std::cout << '\n';
for (auto& i : v) // Modifies v because i is a reference
std::cout << ++i << ' ';
std::cout << '\n';
for (auto&& i : v) // Modifies v because i is a rvalue reference (Am I right?)
std::cout << ++i << ' ';
std::cout << '\n';
for (const auto &i : v) // Wouldn't compile without the /**/ because i is const
std::cout << /*++*/i << ' ';
std::cout << '\n';
}
The output:
Initial values: 0 1 2 3 4 5
1 2 3 4 5 6
1 2 3 4 5 6
2 3 4 5 6 7
2 3 4 5 6 7
Both seem to do the same thing here but I want to know what's the difference between
for (auto& i : v)for (auto&& i : v)
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Answer Source
This answer will probably answer your question, the most relevant part is the following:
auto => will copy the element, but a reference is more efficient
auto& => will only bind to modifiable lvalues
const auto& => will bind to anything but make it const, giving us const_iterator
const auto&& => will bind to rvalues
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