How to parse multipart/form-data from http::request<http::string_body> object?

You can use parser which based on Formidable

https://github.com/FooBarWidget/multipart-parser/tree/master

"multipart_parser.h"

// multipart_parser.h

#ifndef _MULTIPART_PARSER_H_
#define _MULTIPART_PARSER_H_

#include <sys/types.h>
#include <string>
#include <stdexcept>
#include <cstring>

class MultipartParser {
public:
    typedef void (*Callback)(const char* buffer, size_t start, size_t end, void* userData);

private:
    static const char CR = 13;
    static const char LF = 10;
    static const char SPACE = 32;
    static const char HYPHEN = 45;
    static const char COLON = 58;
    static const size_t UNMARKED = (size_t)-1;

    enum State {
        ERROR,
        START,
        START_BOUNDARY,
        HEADER_FIELD_START,
        HEADER_FIELD,
        HEADER_VALUE_START,
        HEADER_VALUE,
        HEADER_VALUE_ALMOST_DONE,
        HEADERS_ALMOST_DONE,
        PART_DATA_START,
        PART_DATA,
        PART_END,
        END
    };

    enum Flags {
        PART_BOUNDARY = 1,
        LAST_BOUNDARY = 2
    };

    std::string boundary;
    const char* boundaryData;
    size_t boundarySize;
    bool boundaryIndex[256];
    char* lookbehind;
    size_t lookbehindSize;
    State state;
    int flags;
    size_t index;
    size_t headerFieldMark;
    size_t headerValueMark;
    size_t partDataMark;
    const char* errorReason;

    void resetCallbacks() {
        onPartBegin = NULL;
        onHeaderField = NULL;
        onHeaderValue = NULL;
        onHeaderEnd = NULL;
        onHeadersEnd = NULL;
        onPartData = NULL;
        onPartEnd = NULL;
        onEnd = NULL;
        userData = NULL;
    }

    void indexBoundary() {
        const char* current;
        const char* end = boundaryData + boundarySize;

        memset(boundaryIndex, 0, sizeof(boundaryIndex));

        for (current = boundaryData; current < end; current++) {
            boundaryIndex[(unsigned char)*current] = true;
        }
    }

    void callback(Callback cb, const char* buffer = NULL, size_t start = UNMARKED,
        size_t end = UNMARKED, bool allowEmpty = false)
    {
        if (start != UNMARKED && start == end && !allowEmpty) {
            return;
        }
        if (cb != NULL) {
            cb(buffer, start, end, userData);
        }
    }

    void dataCallback(Callback cb, size_t& mark, const char* buffer, size_t i, size_t bufferLen,
        bool clear, bool allowEmpty = false)
    {
        if (mark == UNMARKED) {
            return;
        }

        if (!clear) {
            callback(cb, buffer, mark, bufferLen, allowEmpty);
            mark = 0;
        }
        else {
            callback(cb, buffer, mark, i, allowEmpty);
            mark = UNMARKED;
        }
    }

    char lower(char c) const {
        return c | 0x20;
    }

    inline bool isBoundaryChar(char c) const {
        return boundaryIndex[(unsigned char)c];
    }

    bool isHeaderFieldCharacter(char c) const {
        return (c >= 'a' && c <= 'z')
            || (c >= 'A' && c <= 'Z')
            || c == HYPHEN;
    }

    void setError(const char* message) {
        state = ERROR;
        errorReason = message;
    }

    void processPartData(size_t& prevIndex, size_t& index, const char* buffer,
        size_t len, size_t boundaryEnd, size_t& i, char c, State& state, int& flags)
    {
        prevIndex = index;

        if (index == 0) {
            // boyer-moore derived algorithm to safely skip non-boundary data
            while (i + boundarySize <= len) {
                if (isBoundaryChar(buffer[i + boundaryEnd])) {
                    break;
                }

                i += boundarySize;
            }
            if (i == len) {
                return;
            }
            c = buffer[i];
        }

        if (index < boundarySize) {
            if (boundary[index] == c) {
                if (index == 0) {
                    dataCallback(onPartData, partDataMark, buffer, i, len, true);
                }
                index++;
            }
            else {
                index = 0;
            }
        }
        else if (index == boundarySize) {
            index++;
            if (c == CR) {
                // CR = part boundary
                flags |= PART_BOUNDARY;
            }
            else if (c == HYPHEN) {
                // HYPHEN = end boundary
                flags |= LAST_BOUNDARY;
            }
            else {
                index = 0;
            }
        }
        else if (index - 1 == boundarySize) {
            if (flags & PART_BOUNDARY) {
                index = 0;
                if (c == LF) {
                    // unset the PART_BOUNDARY flag
                    flags &= ~PART_BOUNDARY;
                    callback(onPartEnd);
                    callback(onPartBegin);
                    state = HEADER_FIELD_START;
                    return;
                }
            }
            else if (flags & LAST_BOUNDARY) {
                if (c == HYPHEN) {
                    callback(onPartEnd);
                    callback(onEnd);
                    state = END;
                }
                else {
                    index = 0;
                }
            }
            else {
                index = 0;
            }
        }
        else if (index - 2 == boundarySize) {
            if (c == CR) {
                index++;
            }
            else {
                index = 0;
            }
        }
        else if (index - boundarySize == 3) {
            index = 0;
            if (c == LF) {
                callback(onPartEnd);
                callback(onEnd);
                state = END;
                return;
            }
        }

        if (index > 0) {
            // when matching a possible boundary, keep a lookbehind reference
            // in case it turns out to be a false lead
            if (index - 1 >= lookbehindSize) {
                setError("Parser bug: index overflows lookbehind buffer. "
                    "Please send bug report with input file attached.");
                throw std::out_of_range("index overflows lookbehind buffer");
            }
            else if (index - 1 < 0) {
                setError("Parser bug: index underflows lookbehind buffer. "
                    "Please send bug report with input file attached.");
                throw std::out_of_range("index underflows lookbehind buffer");
            }
            lookbehind[index - 1] = c;
        }
        else if (prevIndex > 0) {
            // if our boundary turned out to be rubbish, the captured lookbehind
            // belongs to partData
            callback(onPartData, lookbehind, 0, prevIndex);
            prevIndex = 0;
            partDataMark = i;

            // reconsider the current character even so it interrupted the sequence
            // it could be the beginning of a new sequence
            i--;
        }
    }

public:
    Callback onPartBegin;
    Callback onHeaderField;
    Callback onHeaderValue;
    Callback onHeaderEnd;
    Callback onHeadersEnd;
    Callback onPartData;
    Callback onPartEnd;
    Callback onEnd;
    void* userData;

    MultipartParser() {
        lookbehind = NULL;
        resetCallbacks();
        reset();
    }

    MultipartParser(const std::string& boundary) {
        lookbehind = NULL;
        resetCallbacks();
        setBoundary(boundary);
    }

    ~MultipartParser() {
        delete[] lookbehind;
    }

    void reset() {
        delete[] lookbehind;
        state = ERROR;
        boundary.clear();
        boundaryData = boundary.c_str();
        boundarySize = 0;
        lookbehind = NULL;
        lookbehindSize = 0;
        flags = 0;
        index = 0;
        headerFieldMark = UNMARKED;
        headerValueMark = UNMARKED;
        partDataMark = UNMARKED;
        errorReason = "Parser uninitialized.";
    }

    void setBoundary(const std::string& boundary) {
        reset();
        this->boundary = "\r\n--" + boundary;
        boundaryData = this->boundary.c_str();
        boundarySize = this->boundary.size();
        indexBoundary();
        lookbehind = new char[boundarySize + 8];
        lookbehindSize = boundarySize + 8;
        state = START;
        errorReason = "No error.";
    }

    size_t feed(const char* buffer, size_t len) {
        if (state == ERROR || len == 0) {
            return 0;
        }

        State state = this->state;
        int flags = this->flags;
        size_t prevIndex = this->index;
        size_t index = this->index;
        size_t boundaryEnd = boundarySize - 1;
        size_t i;
        char c, cl;

        for (i = 0; i < len; i++) {
            c = buffer[i];

            switch (state) {
            case ERROR:
                return i;
            case START:
                index = 0;
                state = START_BOUNDARY;
            case START_BOUNDARY:
                if (index == boundarySize - 2) {
                    if (c != CR) {
                        setError("Malformed. Expected CR after boundary.");
                        return i;
                    }
                    index++;
                    break;
                }
                else if (index - 1 == boundarySize - 2) {
                    if (c != LF) {
                        setError("Malformed. Expected LF after boundary CR.");
                        return i;
                    }
                    index = 0;
                    callback(onPartBegin);
                    state = HEADER_FIELD_START;
                    break;
                }
                if (c != boundary[index + 2]) {
                    setError("Malformed. Found different boundary data than the given one.");
                    return i;
                }
                index++;
                break;
            case HEADER_FIELD_START:
                state = HEADER_FIELD;
                headerFieldMark = i;
                index = 0;
            case HEADER_FIELD:
                if (c == CR) {
                    headerFieldMark = UNMARKED;
                    state = HEADERS_ALMOST_DONE;
                    break;
                }

                index++;
                if (c == HYPHEN) {
                    break;
                }

                if (c == COLON) {
                    if (index == 1) {
                        // empty header field
                        setError("Malformed first header name character.");
                        return i;
                    }
                    dataCallback(onHeaderField, headerFieldMark, buffer, i, len, true);
                    state = HEADER_VALUE_START;
                    break;
                }

                cl = lower(c);
                if (cl < 'a' || cl > 'z') {
                    setError("Malformed header name.");
                    return i;
                }
                break;
            case HEADER_VALUE_START:
                if (c == SPACE) {
                    break;
                }

                headerValueMark = i;
                state = HEADER_VALUE;
            case HEADER_VALUE:
                if (c == CR) {
                    dataCallback(onHeaderValue, headerValueMark, buffer, i, len, true, true);
                    callback(onHeaderEnd);
                    state = HEADER_VALUE_ALMOST_DONE;
                }
                break;
            case HEADER_VALUE_ALMOST_DONE:
                if (c != LF) {
                    setError("Malformed header value: LF expected after CR");
                    return i;
                }

                state = HEADER_FIELD_START;
                break;
            case HEADERS_ALMOST_DONE:
                if (c != LF) {
                    setError("Malformed header ending: LF expected after CR");
                    return i;
                }

                callback(onHeadersEnd);
                state = PART_DATA_START;
                break;
            case PART_DATA_START:
                state = PART_DATA;
                partDataMark = i;
            case PART_DATA:
                processPartData(prevIndex, index, buffer, len, boundaryEnd, i, c, state, flags);
                break;
            default:
                return i;
            }
        }

        dataCallback(onHeaderField, headerFieldMark, buffer, i, len, false);
        dataCallback(onHeaderValue, headerValueMark, buffer, i, len, false);
        dataCallback(onPartData, partDataMark, buffer, i, len, false);

        this->index = index;
        this->state = state;
        this->flags = flags;

        return len;
    }

    bool succeeded() const {
        return state == END;
    }

    bool hasError() const {
        return state == ERROR;
    }

    bool stopped() const {
        return state == ERROR || state == END;
    }

    const char* getErrorMessage() const {
        return errorReason;
    }
};

#endif /* _MULTIPART_PARSER_H_ */

#include <iostream>
#include <string>
using namespace std;

static void
onPartBegin(const char* buffer, size_t start, size_t end, void* userData) {
    printf("onPartBegin\n");
}

static void
onHeaderField(const char* buffer, size_t start, size_t end, void* userData) {
    printf("onHeaderField: (%s)\n", string(buffer + start, end - start).c_str());
}

static void
onHeaderValue(const char* buffer, size_t start, size_t end, void* userData) {
    printf("onHeaderValue: (%s)\n", string(buffer + start, end - start).c_str());
}

static void
onPartData(const char* buffer, size_t start, size_t end, void* userData) {
    printf("onPartData: (%s)\n", string(buffer + start, end - start).c_str());
}

static void
onPartEnd(const char* buffer, size_t start, size_t end, void* userData) {
    printf("onPartEnd\n");
}


static void
onEnd(const char* buffer, size_t start, size_t end, void* userData) {
    printf("onEnd\n");
}

static void cbHeaderEnd(const char* buffer, size_t start, size_t end, void* userData) {
    std::cout << "Header end" << std::endl;
}

int main()
{
    // Sample multipart/form-data request body
    std::string boundary = "----WebKitFormBoundary7MA4YWxkTrZu0gW";
    std::string body =
        "------WebKitFormBoundary7MA4YWxkTrZu0gW\r\n"
        "Content-Disposition: form-data; name=\"username\"\r\n"
        "\r\n"
        "test_user\r\n"
        "------WebKitFormBoundary7MA4YWxkTrZu0gW\r\n"
        "Content-Disposition: form-data; name=\"file1\"; filename=\"example.txt\"\r\n"
        "Content-Type: text/plain\r\n"
        "\r\n"
        "This is the content of the file.\r\n"
        "------WebKitFormBoundary7MA4YWxkTrZu0gW--";

    MultipartParser parser;
    parser.onPartBegin = onPartBegin;
    parser.onHeaderField = onHeaderField;
    parser.onHeaderValue = onHeaderValue;
    parser.onHeaderEnd = cbHeaderEnd;
    parser.onPartData = onPartData;
    parser.onPartEnd = onPartEnd;
    parser.onEnd = onEnd;
    parser.userData = &parser;

    parser.setBoundary(boundary);

    const char* buf = body.c_str();
    size_t bufsize = body.size();
    size_t fed = 0;
    do {
        size_t ret = parser.feed(buf + fed, bufsize - fed);
        fed += ret;
    } while (fed < bufsize && !parser.stopped());

    return 0;
}