在 C++ 中使用 switch 語句
本文將介紹幾種在 C++ 中使用 switch 語句的方法。
使用 switch 語句構建多程式碼路徑方案
switch 關鍵字的使用與其他基於塊的語句類似，這些語句將一個給定的表示式作為條件進行評估。唯一不同的是，與 case 關鍵字指定的每個程式碼路徑相比，表示式必須是一個整數。路徑具有以下結構-case label：statement，其中 label 是與 switch 條件中的表示式相比的值。如果表示式與其中一個標籤相匹配，程式執行就會移動到該標籤之後的語句。注意，如果 case 塊沒有以 break; 語句結束，則程式將繼續執行後面所有 case 塊中的語句，直到沒有達到 break; 或 switch 作用域本身結束。
#include <iostream>
using std::cout; using std::endl;
using std::cin; using std::string;
enum WEEK { Monday, Tuesday, Wednesday, Thursday,
Friday, Saturday, Sunday};
int main() {
int week_day;
cout << "Type the weekday (0-6): ";
cin >> week_day;
if (cin.fail()) {
cout << "Error occured when reading from stream!" << endl;
exit(EXIT_FAILURE);
};
switch (week_day) {
case Monday: cout << "Monday\n"; break;
case Tuesday: cout << "Tuesday\n"; break;
case Wednesday: cout << "Wednesday\n"; break;
case Thursday: cout << "Thursday\n"; break;
case Friday: cout << "Friday\n"; break;
case Saturday: cout << "Saturday\n"; break;
case Sunday: cout << "Sunday\n"; break;
default: exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}
使用 default 關鍵字在 switch 中指定一個預設程式碼路徑
default 關鍵字定義了特殊情況下的標籤，如果沒有一個宣告的情況與 switch 表示式相匹配，就會被執行。需要注意的重要一點是，每個標籤塊都需要在結尾處使用 break; 語句來滿足 switch 結構的基本邏輯。請注意，接下來的程式碼示例演示了沒有任何 break 的 switch 語句，這將表現得相當突兀。如果 week_day 值與第一個標籤 Monday 相匹配，那麼每個 cout 語句都會被執行，包括與預設標籤並列的語句。另一方面，如果 week_day 值不匹配任何 case 標籤，那麼從 default 標籤開始的 cout 語句將被執行。
#include <iostream>
using std::cout; using std::endl;
using std::cin; using std::string;
enum WEEK { Monday, Tuesday, Wednesday, Thursday,
Friday, Saturday, Sunday};
int main() {
int week_day;
cout << "Type the weekday (0-6): ";
cin >> week_day;
if (cin.fail()) {
cout << "Error occured when reading from stream!" << endl;
exit(EXIT_FAILURE);
};
switch (week_day) {
case Monday: cout << "Monday\n";
case Tuesday: cout << "Tuesday\n";
case Wednesday: cout << "Wednesday\n";
case Thursday: cout << "Thursday\n";
case Friday: cout << "Friday\n";
default: cout << "Wrong number specified!\n";
case Saturday: cout << "Saturday\n";
case Sunday: cout << "Sunday\n";
}
exit(EXIT_SUCCESS);
}
在不同的程式碼路徑中使用 {} 塊符號來宣告區域性變數
和其他基於塊的語句一樣，switch 體有可以宣告變數的作用域，但 case 要求使用者在每個 case 構造中都有區域性變數。請注意，每個 case 路徑的語句都有共享的作用域，除非在 {} 內的獨立塊內宣告。因此，如果一個變數在一個 case 中被初始化，那麼其他情況下的語句就不能引用同一個變數。下面的例子實現了 switch 語句，其中 case Friday 有一個本地塊，而且裡面的變數不能從其他 case 看到。
#include <iostream>
using std::cout; using std::endl;
using std::cin; using std::string;
enum WEEK { Monday, Tuesday, Wednesday, Thursday,
Friday, Saturday, Sunday};
int main() {
int week_day;
cout << "Type the weekday (0-6): ";
cin >> week_day;
if (cin.fail()) {
cout << "Error occured when reading from stream!" << endl;
exit(EXIT_FAILURE);
};
switch (week_day) {
case Monday: cout << "Monday\n"; break;
case Tuesday: cout << "Tuesday\n"; break;
case Wednesday: cout << "Wednesday\n"; break;
case Thursday: cout << "Thursday\n"; break;
case Friday: {
string str = "Friday";
cout << str << endl;
break;
}
case Saturday: cout << "Saturday\n"; break;
case Sunday: cout << "Sunday\n"; break;
default: exit(EXIT_FAILURE);
}
exit(EXIT_SUCCESS);
}
Founder of DelftStack.com. Jinku has worked in the robotics and automotive industries for over 8 years. He sharpened his coding skills when he needed to do the automatic testing, data collection from remote servers and report creation from the endurance test. He is from an electrical/electronics engineering background but has expanded his interest to embedded electronics, embedded programming and front-/back-end programming.
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