Named parameter

More C++ Idioms Named Parameter

isocpp What is the “Named Parameter Idiom”? ¶ Δ

It’s a fairly useful way to exploit method chaining.

The fundamental problem solved by the Named Parameter Idiom is that C++ only supports positional parameters.

NOTE: 所谓named parameter，其实就是python中的kwargs。

#include <string>

class File;
class OpenFile
{
public:
    OpenFile(const std::string& filename);
    // sets all the default values for each data member
    OpenFile& readonly();  // changes readonly_ to true
    OpenFile& readwrite(); // changes readonly_ to false
    OpenFile& createIfNotExist();
    OpenFile& blockSize(unsigned nbytes);
    // ...
private:
    friend class File;
    std::string filename_;
    bool readonly_;          // defaults to false [for example]
    bool createIfNotExist_;  // defaults to false [for example]
    // ...
    unsigned blockSize_;     // defaults to 4096 [for example]
    // ...
};
inline OpenFile::OpenFile(const std::string& filename)
        : filename_(filename)
                , readonly_(false)
                , createIfNotExist_(false)
                , blockSize_(4096u)
{
}
inline OpenFile& OpenFile::readonly()
{
    readonly_ = true;
    return *this;
}
inline OpenFile& OpenFile::readwrite()
{
    readonly_ = false;
    return *this;
}
inline OpenFile& OpenFile::createIfNotExist()
{
    createIfNotExist_ = true;
    return *this;
}
inline OpenFile& OpenFile::blockSize(unsigned nbytes)
{
    blockSize_ = nbytes;
    return *this;
}

class File
{
public:
    File(const OpenFile& params);
    // ...
};

File::File(const OpenFile& params)
{
    // ...
}

int main()
{
    File f = OpenFile("foo.txt")
            .readonly()
            .createIfNotExist()
            .blockSize(1024);
}

编译g++ tests.cpp

wikipedia Named parameter#With chained method calls

TO READ

https://marcoarena.wordpress.com/2014/12/16/bring-named-parameters-in-modern-cpp/

https://stackoverflow.com/questions/40533193/v8-c-how-to-get-object-key-values-provided-as-arguments

https://www.fluentcpp.com/2018/12/14/named-arguments-cpp/