Qt：switch

港控/mmm° 2023-01-12 10:10 16阅读 0赞

#include <QCoreApplication>
    #ifndef FALLTHROUGH
    #if __has_cpp_attribute(fallthrough)
    #define FALLTHROUGH [[fallthrough]]
    #elif __has_cpp_attribute(clang::fallthrough)
    #define FALLTHROUGH [[clang::fallthrough]]
    #else
    #define FALLTHROUGH
    #endif
    #endif
    
    
    
    int main(int argc, char *argv[])
    {
        
        QCoreApplication a(argc, argv);
    
    
        int n = 1;
        switch (n) {
        
        case 1:
            printf("----------1--------\n");
        case 2:
            printf("----------2--------\n");
        case 3:
            printf("----------3--------\n");
        case 4:
            printf("----------4--------\n");
        default:
            printf("----------o--------\n");
        }
    
        printf("---------j----------");
        return a.exec();
    }

![在这里插入图片描述][20210128101545232.png]

#include <QCoreApplication>
    #ifndef FALLTHROUGH
    #if __has_cpp_attribute(fallthrough)
    #define FALLTHROUGH [[fallthrough]]
    #elif __has_cpp_attribute(clang::fallthrough)
    #define FALLTHROUGH [[clang::fallthrough]]
    #else
    #define FALLTHROUGH
    #endif
    #endif
    
    
    
    int main(int argc, char *argv[])
    {
        
        QCoreApplication a(argc, argv);
    
    
        int n = 1;
        switch (n) {
        
        case 1:
            printf("----------1--------\n");
        case 2:
            printf("----------2--------\n");
            break;
        case 3:
            printf("----------3--------\n");
        case 4:
            printf("----------4--------\n");
        default:
            printf("----------o--------\n");
        }
    
        printf("---------j----------");
        return a.exec();
    }

![在这里插入图片描述][20210128101636631.png]

总结：

*  总结： 如果没有break，switch会一路滑下来，无论条件是否满足

*  break会跳出switch

> 在大多数语言中，switch 语句块中，case 要紧跟 break，否则 case 之后的语句会顺序运行

#include <QCoreApplication>
    #ifndef FALLTHROUGH
    #if __has_cpp_attribute(fallthrough)
    #define FALLTHROUGH [[fallthrough]]
    #elif __has_cpp_attribute(clang::fallthrough)
    #define FALLTHROUGH [[clang::fallthrough]]
    #else
    #define FALLTHROUGH
    #endif
    #endif
    
    
    
    int main(int argc, char *argv[])
    {
        
        QCoreApplication a(argc, argv);
    
    
        int n = 1;
        switch (n) {
        
        case 1:
            printf("----------1--------\n");
        case 2:
            printf("----------2--------\n");
            return 0;
        case 3:
            printf("----------3--------\n");
        case 4:
            printf("----------4--------\n");
        default:
            printf("----------o--------\n");
        }
    
        printf("---------j----------");
        return a.exec();
    }

![在这里插入图片描述][20210128101800531.png]  
总结：return会跳出整个函数

#include <QCoreApplication>
    #ifndef FALLTHROUGH
    #if __has_cpp_attribute(fallthrough)
    #define FALLTHROUGH [[fallthrough]]
    #elif __has_cpp_attribute(clang::fallthrough)
    #define FALLTHROUGH [[clang::fallthrough]]
    #else
    #define FALLTHROUGH
    #endif
    #endif
    
    
    
    int main(int argc, char *argv[])
    {
        
        QCoreApplication a(argc, argv);
    
    
        int n = 1;
        switch (n) {
        
        case 1:
            printf("----------1--------\n");
        FALLTHROUGH; case 2:
            printf("----------2--------\n");
        FALLTHROUGH;case 3:
            printf("----------3--------\n");
        FALLTHROUGH;case 4:
            printf("----------4--------\n");
        FALLTHROUGH; default:
            printf("----------o--------\n");
        }
    
        printf("---------j----------");
        return a.exec();
    }

![在这里插入图片描述][20210128102028833.png]

#include <QCoreApplication>
    #ifndef FALLTHROUGH
    #if __has_cpp_attribute(fallthrough)
    #define FALLTHROUGH [[fallthrough]]
    #elif __has_cpp_attribute(clang::fallthrough)
    #define FALLTHROUGH [[clang::fallthrough]]
    #else
    #define FALLTHROUGH
    #endif
    #endif
    
    
    
    int main(int argc, char *argv[])
    {
        
        QCoreApplication a(argc, argv);
    
    
        int n = 3;
        switch (n) {
        
        case 1:
            printf("----------1--------\n");
            break;
        FALLTHROUGH; case 2:
            printf("----------2--------\n");
            break;
        FALLTHROUGH;case 3:
            printf("----------3--------\n");
            break;
        FALLTHROUGH;case 4:
            printf("----------4--------\n");
            break;
        FALLTHROUGH; default:
            printf("----------o--------\n");
        }
    
        printf("---------j----------");
        return a.exec();
    }

![在这里插入图片描述][20210128102433904.png]

fallthrough 语句让 case 之后的语句会按顺序继续运行，且不论条件是否满足都会执行。 但是break优先级大于fallthrough

[20210128101545232.png]: /images/20221119/3379ce3f8c1e4b73b0c8b1d47c8ec888.png
[20210128101636631.png]: /images/20221119/ab20f18ef0e94ad4b22f8e33fcf8aa0c.png
[20210128101800531.png]: /images/20221119/863dac6fed2442a4a05237522b4ad528.png
[20210128102028833.png]: /images/20221119/0b145c6ec8e4451599765e6b39be2eba.png
[20210128102433904.png]: /images/20221119/d7bb9d2f6cce44ecb891efd02c9340bb.png

Qt：switch

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