Question

当我们需要使用“＆amp;”时什么时候不呢？例如，在下面，两个for循环产生相同的结果。

std::vector< Product* > itemByColor = pF.by_color( vecProds, Color::Red );

for( auto i : itemByColor )
{
     std::cout << " product name <<" << i->name<< std::endl;
}

和

for( auto& i : itemByColor )
{
     std::cout << " product name <<" << i->name<< std::endl;
}

Answer 1

或多或少与您决定键入std::string或（const）std::string&相同。也就是说，您是要复制对象还是要引用它。

std::vector<int> my_vector{ 1, 2, 3, 4, 5 };

int copy = my_vector[ 0 ];
int& reference = my_vector[ 0 ];

++copy;
std::cerr << my_vector[ 0 ] << '\n'; // Outputs '1', since the copy was incremented, not the original object itself

++reference;
std::cerr << my_vector[ 0 ] << '\n'; // Outputs '2', since a reference to the original object was incremented

// For each 'n' in 'my_vector', taken as a copy
for( auto n : my_vector )
{
    // The copy ('n') is modified, but the original remains unaffected
    n = 123;
}

// For each 'n' in 'my_vector', taken as a reference
for( auto& n : my_vector )
{
    // The original is incremented by 42, since 'n' is a reference to it
    n += 42;
}

// At this point, 'my_vector' contains '{ 44, 44, 45, 46, 47 }'

使用“＆amp;”与汽车

1 个答案: