Is it possible to declare two variables of different types in a for loop?

Is it possible to declare two variables of different types in the initialization body of a for loop in C++?

For example:

for(int i=0,j=0 ...

defines two integers. Can I define an int and a char in the initialization body? How would this be done?

C++17: Yes! You should use a structured binding declaration. The syntax has been supported in gcc and clang since gcc-7 and clang-4.0 (clang live example). This allows us to unpack a tuple like so:

for (auto [i, f, s] = std::tuple{1, 1.0, std::string{"ab"}}; i < N; ++i, f += 1.5) {
    // ...
}

The above will give you:

• int i set to 1

• double f set to 1.0

• std::string s set to "ab"

Make sure to #include for this kind of declaration.

You can specify the exact types inside the tuple by typing them all out as I have with the std::string, if you want to name a type. For example:

auto [vec, i32] = std::tuple{std::vector{3, 4, 5}, std::int32_t{12}}

A specific application of this is iterating over a map, getting the key and value,

std::unordered_map m = { /*...*/ };
for (auto& [key, value] : m) {
   // ...
}

See a live example here

C++14: You can do the same as C++11 (below) with the addition of type-based std::get. So instead of std::get(t) in the below example, you can have std::get(t).

C++11: std::make_pair allows you to do this, as well as std::make_tuple for more than two objects.

for (auto p = std::make_pair(5, std::string("Hello World")); p.first < 10; ++p.first) {
    std::cout << p.second << '\n';
}

std::make_pair will return the two arguments in a std::pair. The elements can be accessed with .first and .second.

For more than two objects, you'll need to use a std::tuple

for (auto t = std::make_tuple(0, std::string("Hello world"), std::vector{});
        std::get(t) (t)) {
    std::cout (t) (t).push_back(std::get(t)); // add counter value to the vector
}

std::make_tuple is a variadic template that will construct a tuple of any number of arguments (with some technical limitations of course). The elements can be accessed by index with std::get(tuple_object)

Within the for loop bodies you can easily alias the objects, though you still need to use .first or std::get for the for loop condition and update expression

for (auto t = std::make_tuple(0, std::string("Hello world"), std::vector{});
        std::get(t) (t)) {
    auto& i = std::get(t);
    auto& s = std::get(t);
    auto& v = std::get(t);
    std::cout << s << '\n'; // cout Hello world
    v.push_back(i); // add counter value to the vector
}

C++98 and C++03 You can explicitly name the types of a std::pair. There is no standard way to generalize this to more than two types though:

for (std::pair p(5, "Hello World"); p.first < 10; ++p.first) {
    std::cout << p.second << '\n';
}

Stack Overflow Q#2687392, score: 257