c++ - 不理解这段代码(无条件运行)

Question

我正在学习 C++，但在我读过的任何一本书中都没有真正看到这一点。我想阅读和评论代码，以便我可以更好地学习，并遇到了一段奇怪的代码，它运行但没有条件。根据我阅读的内容(以及我使用其他语言的经验，您需要一个 if、while、for 或 something for blocks)。

我正在查看 tbb 线程包，所以我不确定它是否与启动线程或特定于 c++ 相关(如果您不认为这是 c++ 中常见的东西，那么它可能特定于 tdd)。

我想我明白里面的代码实际上做了什么，但我不确定它是如何被触发或运行的。有任何想法吗？

这是部分:

    {
        //this is the graph part of the code
        Graph g;
        g.create_random_dag(nodes);
        std::vector<Cell*> root_set;
        g.get_root_set(root_set);
        root_set_size = root_set.size();
        for( unsigned int trial=0; trial<traversals; ++trial ) {
            ParallelPreorderTraversal(root_set);
        }
    }

附注如果对您有帮助，这里是整个文件(上面的代码在 main() 的中间)。

#include <cstdlib>
#include "tbb/task_scheduler_init.h"
#include "tbb/tick_count.h"
#include "../../common/utility/utility.h"
#include <iostream>
#include <vector>
#include "Graph.h"

// some forward declarations
class Cell;
void ParallelPreorderTraversal( const std::vector<Cell*>& root_set );

//------------------------------------------------------------------------
// Test driver
//------------------------------------------------------------------------
utility::thread_number_range threads(tbb::task_scheduler_init::default_num_threads);
static unsigned nodes = 1000;
static unsigned traversals = 500;
static bool SilentFlag = false;

//! Parse the command line.
static void ParseCommandLine( int argc, const char* argv[] ) {
    utility::parse_cli_arguments(
            argc,argv,
            utility::cli_argument_pack()
                //"-h" option for for displaying help is present implicitly
                .positional_arg(threads,"n-of-threads","number of threads to use; a range of the form low[:high], where low and optional high are non-negative integers or 'auto' for the TBB default.")
                .positional_arg(nodes,"n-of-nodes","number of nodes in the graph.")
                .positional_arg(traversals,"n-of-traversals","number of times to evaluate the graph. Reduce it (e.g. to 100) to shorten example run time\n")
                .arg(SilentFlag,"silent","no output except elapsed time ")
    );
}

int main( int argc, const char* argv[] ) {
    try {
        tbb::tick_count main_start = tbb::tick_count::now(); //tbb counter start
        ParseCommandLine(argc,argv);

        // Start scheduler with given number of threads.
        std::cout << threads << std::endl;
        for( int p=threads.first; p<=threads.last; ++p ) {
            tbb::tick_count t0 = tbb::tick_count::now(); //timer
            tbb::task_scheduler_init init(4); //creates P number of threads
            srand(2); //generates a random number between 0-2?
            size_t root_set_size = 0; 
            {
                //this is the graph part of the code
                Graph g;
                g.create_random_dag(nodes);
                std::vector<Cell*> root_set;
                g.get_root_set(root_set);
                root_set_size = root_set.size();
                for( unsigned int trial=0; trial<traversals; ++trial ) {
                    ParallelPreorderTraversal(root_set);
                }
            }
            tbb::tick_count::interval_t interval = tbb::tick_count::now()-t0; //counter done
            if (!SilentFlag){ //output the results
                std::cout
                    <<interval.seconds()<<" seconds using "<<p<<" threads ("<<root_set_size<<" nodes in root_set)\n";
            }
        }
        utility::report_elapsed_time((tbb::tick_count::now()-main_start).seconds());

        return 0;
    }catch(std::exception& e){
        std::cerr
            << "unexpected error occurred. \n"
            << "error description: "<<e.what()<<std::endl;
        return -1;
    }
}

Answer 1

不，您不需要 if 或 while 语句来引入新级别的范围。基本上 { 符号打开一个新的范围级别，} 结束它。通常的作用域规则适用，例如，在这个新 block 中定义的变量在外部未定义，在 block 结束时运行对象析构函数，并且在上面的作用域级别中与另一个名称相同的变量将被隐藏。

一个常见的用例是在 switch 语句中。例如，

switch (a)
{
    case 1:
    {
        int i;
    }
    case 2:
    {
        int i; //note reuse of variable with the same name as in case 1
    }
}

如果 case 语句中没有 { }，编译器将提示多重定义的标识符。