网上找的，忘了原地址

#include <stdint.h>
#include <chrono>
#include <string>
#include <stdio.h>
using namespace std;

static const char* encode_chars[2] = {
             "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
             "abcdefghijklmnopqrstuvwxyz"
             "0123456789"
             "+/",
             "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
             "abcdefghijklmnopqrstuvwxyz"
             "0123456789"
             "-_"};
// Avoiding name mangling
extern "C" {
//转码base64
   __attribute__((visibility("default"))) __attribute__((used))
  const char* encode_base64(unsigned char const* bytes_to_encode, size_t in_len, bool url) {
       size_t len_encoded = (in_len +2) / 3 * 4;
       unsigned char trailing_char = url ? '.' : '=';
 //
 // Choose set of base64 characters. They differ
 // for the last two positions, depending on the url
 // parameter.
 // A bool (as is the parameter url) is guaranteed
 // to evaluate to either 0 or 1 in C++ therefore,
 // the correct character set is chosen by subscripting
 // base64_chars with url.
 //
       const char* base64_chars_s = encode_chars[url];
       std::string ret;
       ret.reserve(len_encoded);
       unsigned int pos = 0;
       while (pos < in_len) {
          ret.push_back(base64_chars_s[(bytes_to_encode[pos + 0] & 0xfc) >> 2]);
          if (pos+1 < in_len) {
             ret.push_back(base64_chars_s[((bytes_to_encode[pos + 0] & 0x03) << 4) + ((bytes_to_encode[pos + 1] & 0xf0) >> 4)]);
             if (pos+2 < in_len) {
                ret.push_back(base64_chars_s[((bytes_to_encode[pos + 1] & 0x0f) << 2) + ((bytes_to_encode[pos + 2] & 0xc0) >> 6)]);
                ret.push_back(base64_chars_s[  bytes_to_encode[pos + 2] & 0x3f]);
             }
             else {
                ret.push_back(base64_chars_s[(bytes_to_encode[pos + 1] & 0x0f) << 2]);
                ret.push_back(trailing_char);
             }
          }
          else {
              ret.push_back(base64_chars_s[(bytes_to_encode[pos + 0] & 0x03) << 4]);
              ret.push_back(trailing_char);
              ret.push_back(trailing_char);
          }
          pos += 3;
    }
    const char *p = ret.c_str();
    return p;
}
static unsigned int pos_of_char(const unsigned char chr) {
 //
 // Return the position of chr within base64_encode()
 //
    if      (chr >= 'A' && chr <= 'Z') return chr - 'A';
    else if (chr >= 'a' && chr <= 'z') return chr - 'a' + ('Z' - 'A')               + 1;
    else if (chr >= '0' && chr <= '9') return chr - '0' + ('Z' - 'A') + ('z' - 'a') + 2;
    else if (chr == '+' || chr == '-') return 62; // Be liberal with input and accept both url ('-') and non-url ('+') base 64 characters (
    else if (chr == '/' || chr == '_') return 63; // Ditto for '/' and '_'
    else
 //
 // 2020-10-23: Throw std::exception rather than const char*
 //(Pablo Martin-Gomez, https://github.com/Bouska)
 //
    throw std::runtime_error("Input is not valid base64-encoded data.");
}
static std::string decode_base64(String encoded_string) {
    if (encoded_string.empty()) return std::string();
    size_t length_of_string = encoded_string.length();
    size_t pos = 0;
 //
 // The approximate length (bytes) of the decoded string might be one or
 // two bytes smaller, depending on the amount of trailing equal signs
 // in the encoded string. This approximation is needed to reserve
 // enough space in the string to be returned.
 //
    size_t approx_length_of_decoded_string = length_of_string / 4 * 3;
    std::string ret;
    ret.reserve(approx_length_of_decoded_string);
    while (pos < length_of_string) {
    //
    // Iterate over encoded input string in chunks. The size of all
    // chunks except the last one is 4 bytes.
    //
    // The last chunk might be padded with equal signs or dots
    // in order to make it 4 bytes in size as well, but this
    // is not required as per RFC 2045.
    //
    // All chunks except the last one produce three output bytes.
    //
    // The last chunk produces at least one and up to three bytes.
    //
       size_t pos_of_char_1 = pos_of_char(encoded_string[pos+1] );
    //
    // Emit the first output byte that is produced in each chunk:
    //
       ret.push_back(static_cast<std::string::value_type>( ( (pos_of_char(encoded_string[pos+0]) ) << 2 ) + ( (pos_of_char_1 & 0x30 ) >> 4)));
       if ( ( pos + 2 < length_of_string  )       &&  // Check for data that is not padded with equal signs (which is allowed by RFC 2045)
              encoded_string[pos+2] != '='        &&
              encoded_string[pos+2] != '.'            // accept URL-safe base 64 strings, too, so check for '.' also.
          )
       {
       //
       // Emit a chunk's second byte (which might not be produced in the last chunk).
       //
          unsigned int pos_of_char_2 = pos_of_char(encoded_string[pos+2] );
          ret.push_back(static_cast<std::string::value_type>( (( pos_of_char_1 & 0x0f) << 4) + (( pos_of_char_2 & 0x3c) >> 2)));
          if ( ( pos + 3 < length_of_string )     &&
                 encoded_string[pos+3] != '='     &&
                 encoded_string[pos+3] != '.'
             )
          {
          //
          // Emit a chunk's third byte (which might not be produced in the last chunk).
          //
             ret.push_back(static_cast<std::string::value_type>( ( (pos_of_char_2 & 0x03 ) << 6 ) + pos_of_char(encoded_string[pos+3])   ));
          }
       }
       pos += 4;
    }
    return ret;
}

调用

    void baseStr2Mat(char *base64Str){
        std::string str = base64Str;
        string decoded_string = decode_base64(str);
        vector<uchar> data(decoded_string.begin(), decoded_string.end());
       //..拿到data去转换 如: Mat mat = imdecode(data, IMREAD_UNCHANGED);
    }
  
   const char* mat2BaseStr(char* imagePath){
         Mat srcMat;
         srcImage = imread(inputImagePath,IMREAD_UNCHANGED);
         std::vector<uchar> buf;
        //图片转成buf 如：cv::imencode(".jpg", srcMat, buf);
         auto *msg = reinterpret_cast<unsigned char*>(buf.data());
        const char* p = encode_base64(msg, buf.size(),false);
        return p;
      }

flutter读取base64图片

Image.memory(
           base64Decode(base64Str),
           //防止重绘
           gaplessPlayback: true,
           width: 100,
           height: 100,
           fit: BoxFit.cover,
         ),