問題描述

是否可以在 C++ 中 for 循環的初始化體中聲明兩個不同類型的變量?

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

例如:

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

定義兩個整數.我可以在初始化體中定義一個 int 和一個 char 嗎?這將如何完成?

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

推薦答案

C++17:是的！您應該使用 結構化綁定聲明.自 gcc-7 和 clang-4.0(clang live example)以來，gcc 和 clang 已支持該語法.這允許我們像這樣解包一個元組:

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) {
    // ...
}

以上會給你:

• int i 設置為 1
• double f 設置為 1.0
• std::string s 設置為 "ab"

• int i set to 1
• double f set to 1.0
• std::string s set to "ab"

對于這種聲明，請確保#include .

Make sure to #include <tuple> for this kind of declaration.

如果你想命名一個類型，你可以在 tuple 中指定確切的類型，就像我在 std::string 中所做的那樣.例如:

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<int>{3, 4, 5}, std::int32_t{12}}

它的一個具體應用是迭代地圖，獲取鍵和值，

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

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

查看現場示例此處

C++14:您可以通過添加基于類型的 std::get 來執行與 C++11(如下)相同的操作.因此，在下面的示例中，您可以使用 std::get(t)，而不是 std::get<0>(t).

C++14: You can do the same as C++11 (below) with the addition of type-based std::get. So instead of std::get<0>(t) in the below example, you can have std::get<int>(t).

C++11:std::make_pair 允許您這樣做，以及 std::make_tuple 用于兩個以上的對象.

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 << std::endl;
}

std::make_pair 將返回 std::pair 中的兩個參數.元素可以通過 .first 和 .second 訪問.

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

對于兩個以上的對象，您需要使用 std::tuple

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<int>{});
        std::get<0>(t) < 10;
        ++std::get<0>(t)) {
    std::cout << std::get<1>(t) << std::endl; // cout Hello world
    std::get<2>(t).push_back(std::get<0>(t)); // add counter value to the vector
}

std::make_tuple 是一個可變參數模板，它將構造一個包含任意數量參數的元組(當然有一些技術限制).可以使用 std::get(tuple_object)

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<INDEX>(tuple_object)

在 for 循環體中，您可以輕松地為對象設置別名，但您仍然需要使用 .first 或 std::get 作為 for 循環條件和更新表達式

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<int>{});
        std::get<0>(t) < 10;
        ++std::get<0>(t)) {
    auto& i = std::get<0>(t);
    auto& s = std::get<1>(t);
    auto& v = std::get<2>(t);
    std::cout << s << std::endl; // cout Hello world
    v.push_back(i); // add counter value to the vector
}

C++98 和 C++03 您可以明確命名 std::pair 的類型.雖然沒有標準方法可以將其概括為兩種以上類型:

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<int, std::string> p(5, "Hello World"); p.first < 10; ++p.first) {
    std::cout << p.second << std::endl;
}

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