无法编译 C++ 函数

Question

My problem might be a petty one, but as a beginner I want to learn the details and hence would be thankful for anyone pointing out my mistakes.

In my header file, I have defined a function like this

void  GetFileNames(const fs::path&, const string&, vector<fs::path>&, const bool);

Then the function definition goes like this in GetFileNames.cpp :

void GetFileNames(const fs::path& root, const string& ext, vector<fs::path>& names, const bool recursiveflag)
{
        if(!fs::exists(root) || !fs::is_directory(root))
        {
                cout<<"The root path either does not exist, or is not a valid folder.\n"<<endl;
                return;
        }

    if (recursiveflag)
    {
            fs::recursive_directory_iterator it(root);
            fs::recursive_directory_iterator endit;
    }
    else
    {
            fs::directory_iterator it(root);
            fs::directory_iterator endit;
    }

    while(it != endit)
    {
            if(fs::is_regular_file(*it) && it->path().extension() == ext)
                    names.push_back(it->path().filename());

            ++it;
    }
}

When I then try to compile program test.cpp where the function has been called in following way like this :

int main(int argc, char** argv)
{
        vector<fs::path> names;
        fs::path root(argv[1]);
        string ext(argv[2]);
        GetFileNames(root,ext,names,true);
        for(auto i = names.begin(); i!= names.end(); ++i)
                cout<< (*i).string()<<endl;
        return 0;
}

I receive the following error :

Scanning dependencies of target Test
[ 33%] Building CXX object src/CMakeFiles/Test.dir/test.cpp.o
[ 66%] Building CXX object src/CMakeFiles/Test.dir/GetFileNames.cpp.o
/Users/ujjwalujjwal/prog/src/GetFileNames.cpp:22:8: error: use of undeclared identifier 'it'
        while(it != endit)
              ^
/Users/ujjwalujjwal/prog/src/GetFileNames.cpp:22:14: error: use of undeclared identifier 'endit'
        while(it != endit)
                    ^
/Users/ujjwalujjwal/prog/src/GetFileNames.cpp:24:27: error: use of undeclared identifier 'it'
                if(fs::is_regular_file(*it) && it->path().extension() == ext)
                                        ^
/Users/ujjwalujjwal/prog/src/GetFileNames.cpp:24:34: error: use of undeclared identifier 'it'
                if(fs::is_regular_file(*it) && it->path().extension() == ext)
                                               ^
/Users/ujjwalujjwal/prog/src/GetFileNames.cpp:25:20: error: use of undeclared identifier 'it'
                        names.push_back(it->path().filename());
                                        ^
/Users/ujjwalujjwal/prog/src/GetFileNames.cpp:27:5: error: use of undeclared identifier 'it'
                ++it;
                  ^
6 errors generated.
make[2]: *** [src/CMakeFiles/Test.dir/GetFileNames.cpp.o] Error 1
make[1]: *** [src/CMakeFiles/Test.dir/all] Error 2
make: *** [all] Error 2

I do not seem to understand that where exactly I am making a conceptual error. This seems very trivial, but I would like to understand whats going on here.

Answer 1

You are defining the iterators within a prior if-else scope:

fs::recursive_directory_iterator it(root);
fs::recursive_directory_iterator endit;

Therefore when you reference them later in the while loop they no longer exist as the variables are destroyed as soon as your if-else block is finished.

As you have iterators that are of different types depending on the bool argument, you might have to actually place the while loop within each of your if and else blocks to ensure that the iterators still exist when you execute your while loop.

However you choose to proceed it is nothing to do with the boolean parameter to your function - pay attention to the compiler errors as they are telling you what is wrong.

Answer 2

it是在另一个块中定义的，你不能在你的 while 循环中使用它，除非你之前定义它

Answer 3

The problem is this bit of code...

    ....
    while(it != endit)
    {
        if(fs::is_regular_file(*it) && it->path().extension() == ext)
                names.push_back(it->path().filename());

        ++it;
    }
    ....

...is using two variables, 'it' and 'endit'. However in both cases where you declare these are inside {} blocks. This makes the declaration only local to these blocks.

Your problem is made slightly worse by each declaration of 'it' and 'endit' being of different types.

You can either move the declaration outside of the local blocks and make them of some common type (in this case InputIterators) or you can move the while loop at the end inside both local blocks like this...

...
    if (recursiveflag)
    {
        fs::recursive_directory_iterator it(root);
        fs::recursive_directory_iterator endit;

        while(it != endit)
        {
            if(fs::is_regular_file(*it) && it->path().extension() == ext)
                    names.push_back(it->path().filename());

            ++it;
        }
    }
    else
    {
        fs::directory_iterator it(root);
        fs::directory_iterator endit;

        while(it != endit)
        {
            if(fs::is_regular_file(*it) && it->path().extension() == ext)
                    names.push_back(it->path().filename());

            ++it;
        }
    }
...

Although this leads to unnecessary code duplication and makes for maintenance nightmares further on.