c++17 的 std::ptr_fun 替换

Question

I am using std::ptr_fun as follows:

static inline std::string &ltrim(std::string &s) {
    s.erase(s.begin(), std::find_if(s.begin(), s.end(), std::not1(std::ptr_fun<int, int>(std::isspace))));
    return s;
}

as presented in this answer .

However this does not compile with C++17 (using Microsoft Visual Studio 2017), with the error:

error C2039: 'ptr_fun': is not a member of 'std'

How can this be fixed?

Answer 1

You use a lambda:

static inline std::string &ltrim(std::string &s) {
    s.erase(s.begin(), std::find_if(s.begin(), s.end(), [](int c) {return !std::isspace(c);}));
    return s;
}

The answer you cited is from 2008, well before C++11 and lambdas existed.

Answer 2

Just use a lambda:

[](unsigned char c){ return !std::isspace(c); }

Note that I changed the argument type to unsigned char , see the notes for std::isspace for why.

std::ptr_fun was deprecated in C++11, and will be removed completely in C++17.

Answer 3

According to cppreference , std::ptr_fun is deprecated since C++11 and discontinued since C++17.

Similarly, std::not1 is deprecated since C++17.

So best don't use either, but a lambda (as explained in other answers).

Answer 4

Alternatively, you might use std::not_fn :

static inline std::string &ltrim(std::string &s) {
    s.erase(s.begin(), std::find_if(s.begin(), s.end(),
        std::not_fn(static_cast<int(*)(int)>(std::isspace))));
    return s;
}

Answer 5

My answer is similar to the already given answers in this thread. But instead of

int isspace(int c);

function from the standard C library, I am suggesting to use

bool isspace(char c, const locale& loc);

function instantiation from the standard C++ library ( http://en.cppreference.com/w/cpp/locale/isspace ), which is more type-correct. In this case you don't need to think about char -> unsigned char -> int conversions and about the current user's locale.

The lambda which searches for non-space will looks like this then:

[](char c) { return !std::isspace(c, std::locale::classic()); }

And the full code of ltrim function will look like this:

static inline std::string& ltrim(std::string& s) {
    auto is_not_space = [](char c) { return !std::isspace(c, std::locale::classic()); };
    auto first_non_space = std::find_if(s.begin(), s.end(), is_not_space);
    s.erase(s.begin(), first_non_space);
    return s;
}

Answer 6

You use Lambda as suggested by Nicol Bolas but you can use auto and type will be deduced there, as follow:-

    static inline std::string &ltrim(std::string &s) {
    s.erase(s.begin(), std::find_if(s.begin(), s.end(), [](auto c) {return 
       !std::isspace(c);}));
    return s;
  }