Why is splitting a string slower in C++ than Python?

I suspect that this is related to buffering on sys.stdin in Python, but no buffering in the C++ implementation.

See this post for details on how to change the buffer size, then try the comparison again:
Setting smaller buffer size for sys.stdin?

I suspect that this is because of the way std::vector gets resized during the process of a push_back() function call. If you try using std::list or std::vector::reserve() to reserve enough space for the sentences, you should get a much better performance. Or you could use a combination of both like below for split1():

void split1(vector<string> &tokens, const string &str,
        const string &delimiters = " ") {
    // Skip delimiters at beginning
    string::size_type lastPos = str.find_first_not_of(delimiters, 0);

    // Find first non-delimiter
    string::size_type pos = str.find_first_of(delimiters, lastPos);
    list<string> token_list;

    while (string::npos != pos || string::npos != lastPos) {
        // Found a token, add it to the list
        token_list.push_back(str.substr(lastPos, pos - lastPos));
        // Skip delimiters
        lastPos = str.find_first_not_of(delimiters, pos);
        // Find next non-delimiter
        pos = str.find_first_of(delimiters, lastPos);
    }
    tokens.assign(token_list.begin(), token_list.end());
}

EDIT: The other obvious thing I see is that Python variable dummy gets assigned each time but not modified. So it’s not a fair comparison against C++. You should try modifying your Python code to be dummy = [] to initialize it and then do dummy += line.split(). Can you report the runtime after this?

EDIT2: To make it even more fair can you modify the while loop in C++ code to be:

    while(cin) {
        getline(cin, input_line);
        std::vector<string> spline; // create a new vector

        //I'm trying one of the two implementations, per compilation, obviously:
//        split1(spline, input_line);  
        split2(spline, input_line);

        count++;
    };

You’re making the mistaken assumption that your chosen C++ implementation is necessarily faster than Python’s. String handling in Python is highly optimized. See this question for more: Why do std::string operations perform poorly?

void split5(vector<string> &tokens, const string &str, char delim=' ') {

    enum { do_token, do_delim } state = do_delim;
    int idx = 0, tok_start = 0;
    for (string::const_iterator it = str.begin() ; ; ++it, ++idx) {
        switch (state) {
            case do_token:
                if (it == str.end()) {
                    tokens.push_back (str.substr(tok_start, idx-tok_start));
                    return;
                }
                else if (*it == delim) {
                    state = do_delim;
                    tokens.push_back (str.substr(tok_start, idx-tok_start));
                }
                break;

            case do_delim:
                if (it == str.end()) {
                    return;
                }
                if (*it != delim) {
                    state = do_token;
                    tok_start = idx;
                }
                break;
        }
    }
}

I’m not providing any better solutions (at least performance-wise), but some additional data that could be interesting.

Using strtok_r (reentrant variant of strtok):

void splitc1(vector<string> &tokens, const string &str,
        const string &delimiters = " ") {
    char *saveptr;
    char *cpy, *token;

    cpy = (char*)malloc(str.size() + 1);
    strcpy(cpy, str.c_str());

    for(token = strtok_r(cpy, delimiters.c_str(), &saveptr);
        token != NULL;
        token = strtok_r(NULL, delimiters.c_str(), &saveptr)) {
        tokens.push_back(string(token));
    }

    free(cpy);
}

Additionally using character strings for parameters, and fgets for input:

void splitc2(vector<string> &tokens, const char *str,
        const char *delimiters) {
    char *saveptr;
    char *cpy, *token;

    cpy = (char*)malloc(strlen(str) + 1);
    strcpy(cpy, str);

    for(token = strtok_r(cpy, delimiters, &saveptr);
        token != NULL;
        token = strtok_r(NULL, delimiters, &saveptr)) {
        tokens.push_back(string(token));
    }

    free(cpy);
}

And, in some cases, where destroying the input string is acceptable:

void splitc3(vector<string> &tokens, char *str,
        const char *delimiters) {
    char *saveptr;
    char *token;

    for(token = strtok_r(str, delimiters, &saveptr);
        token != NULL;
        token = strtok_r(NULL, delimiters, &saveptr)) {
        tokens.push_back(string(token));
    }
}

The timings for these are as follows (including my results for the other variants from the question and the accepted answer):

split1.cpp:  C++   : Saw 20000000 lines in 31 seconds.  Crunch speed: 645161
split2.cpp:  C++   : Saw 20000000 lines in 45 seconds.  Crunch speed: 444444
split.py:    Python: Saw 20000000 lines in 33 seconds.  Crunch Speed: 606060
split5.py:   Python: Saw 20000000 lines in 35 seconds.  Crunch Speed: 571428
split6.cpp:  C++   : Saw 20000000 lines in 18 seconds.  Crunch speed: 1111111

splitc1.cpp: C++   : Saw 20000000 lines in 27 seconds.  Crunch speed: 740740
splitc2.cpp: C++   : Saw 20000000 lines in 22 seconds.  Crunch speed: 909090
splitc3.cpp: C++   : Saw 20000000 lines in 20 seconds.  Crunch speed: 1000000

As we can see, the solution from the accepted answer is still fastest.

For anyone who would want to do further tests, I also put up a Github repo with all the programs from the question, the accepted answer, this answer, and additionally a Makefile and a script to generate test data: https://github.com/tobbez/string-splitting.

If you take the split1 implementaion and change the signature to more closely match that of split2, by changing this:

void split1(vector<string> &tokens, const string &str, const string &delimiters = " ")

to this:

void split1(vector<string> &tokens, const string &str, const char delimiters = ' ')

You get a more dramatic difference between split1 and split2, and a fairer comparison:

split1  C++   : Saw 10000000 lines in 41 seconds.  Crunch speed: 243902
split2  C++   : Saw 10000000 lines in 144 seconds.  Crunch speed: 69444
split1' C++   : Saw 10000000 lines in 33 seconds.  Crunch speed: 303030

I think the following code is better, using some C++17 and C++14 features:

// These codes are un-tested when I write this post, but I'll test it
// When I'm free, and I sincerely welcome others to test and modify this
// code.

// C++17
#include <istream>     // For std::istream.
#include <string_view> // new feature in C++17, sizeof(std::string_view) == 16 in libc++ on my x86-64 debian 9.4 computer.
#include <string>
#include <utility>     // C++14 feature std::move.

template <template <class...> class Container, class Allocator>
void split1(Container<std::string_view, Allocator> &tokens, 
            std::string_view str,
            std::string_view delimiter = " ") 
{
    /* 
     * The model of the input string:
     *
     * (optional) delimiter | content | delimiter | content | delimiter| 
     * ... | delimiter | content 
     *
     * Using std::string::find_first_not_of or 
     * std::string_view::find_first_not_of is a bad idea, because it 
     * actually does the following thing:
     * 
     *     Finds the first character not equal to any of the characters 
     *     in the given character sequence.
     * 
     * Which means it does not treeat your delimiters as a whole, but as
     * a group of characters.
     * 
     * This has 2 effects:
     *
     *  1. When your delimiters is not a single character, this function
     *  won't behave as you predicted.
     *
     *  2. When your delimiters is just a single character, the function
     *  may have an additional overhead due to the fact that it has to 
     *  check every character with a range of characters, although 
     * there's only one, but in order to assure the correctness, it still 
     * has an inner loop, which adds to the overhead.
     *
     * So, as a solution, I wrote the following code.
     *
     * The code below will skip the first delimiter prefix.
     * However, if there's nothing between 2 delimiter, this code'll 
     * still treat as if there's sth. there.
     *
     * Note: 
     * Here I use C++ std version of substring search algorithm, but u
     * can change it to Boyer-Moore, KMP(takes additional memory), 
     * Rabin-Karp and other algorithm to speed your code.
     * 
     */

    // Establish the loop invariant 1.
    typename std::string_view::size_type 
        next, 
        delimiter_size = delimiter.size(),  
        pos = str.find(delimiter) ? 0 : delimiter_size;

    // The loop invariant:
    //  1. At pos, it is the content that should be saved.
    //  2. The next pos of delimiter is stored in next, which could be 0
    //  or std::string_view::npos.

    do {
        // Find the next delimiter, maintain loop invariant 2.
        next = str.find(delimiter, pos);

        // Found a token, add it to the vector
        tokens.push_back(str.substr(pos, next));

        // Skip delimiters, maintain the loop invariant 1.
        //
        // @ next is the size of the just pushed token.
        // Because when next == std::string_view::npos, the loop will
        // terminate, so it doesn't matter even if the following 
        // expression have undefined behavior due to the overflow of 
        // argument.
        pos = next + delimiter_size;
    } while(next != std::string_view::npos);
}   

template <template <class...> class Container, class traits, class Allocator2, class Allocator>
void split2(Container<std::basic_string<char, traits, Allocator2>, Allocator> &tokens, 
            std::istream &stream,
            char delimiter = ' ')
{
    std::string<char, traits, Allocator2> item;

    // Unfortunately, std::getline can only accept a single-character 
    // delimiter.
    while(std::getline(stream, item, delimiter))
        // Move item into token. I haven't checked whether item can be 
        // reused after being moved.
        tokens.push_back(std::move(item));
}

The choice of container:

1. std::vector.

   Assuming the initial size of allocated internal array is 1, and the ultimate size is N, you will allocate and deallocate for log2(N) times, and you will copy the (2 ^ (log2(N) + 1) – 1) = (2N – 1) times. As pointed out in Is the poor performance of std::vector due to not calling realloc a logarithmic number of times?, this can have a poor performance when the size of vector is unpredictable and could be very large.
   But, if you can estimate the size of it, this’ll be less a problem.

2. std::list.

   For every push_back, the time it consumed is a constant, but it’ll probably takes more time than std::vector on individual push_back. Using a per-thread memory pool and a custom allocator can ease this problem.

3. std::forward_list.

   Same as std::list, but occupy less memory per element. Require a wrapper class to work due to the lack of API push_back.

4. std::array.

   If you can know the limit of growth, then you can use std::array. Of cause, you can’t use it directly, since it doesn’t have the API push_back. But you can define a wrapper, and I think it’s the fastest way here and can save some memory if your estimation is quite accurate.

5. std::deque.

   This option allows you to trade memory for performance. There’ll be no (2 ^ (N + 1) – 1) times copy of element, just N times allocation, and no deallocation. Also, you’ll has constant random access time, and the ability to add new elements at both ends.

According to std::deque-cppreference

On the other hand, deques typically have large minimal memory cost; a
deque holding just one element has to allocate its full internal array
(e.g. 8 times the object size on 64-bit libstdc++; 16 times the object size
or 4096 bytes, whichever is larger, on 64-bit libc++)

or you can use combo of these:

1. std::vector< std::array<T, 2 ^ M> >

   This is similar to std::deque, the difference is just this container doesn’t support to add element at the front. But it is still faster in performance, due to the fact that it won’t copy the underlying std::array for (2 ^ (N + 1) – 1) times, it’ll just copy the pointer array for (2 ^ (N – M + 1) – 1) times, and allocating new array only when the current is full and doesn’t need to deallocate anything. By the way, you can get constant random access time.

2. std::list< std::array<T, ...> >

   Greatly ease the pressure of memory framentation. It will only allocate new array when the current is full, and does not need to copy anything. You will still have to pay the price for an additional pointer conpared to combo 1.

3. std::forward_list< std::array<T, ...> >

   Same as 2, but cost the same memory as combo 1.