How do I use stringstream while loop logic for whole text file in C++

Question

The following links I have used to helped get closer to a solution, however I am still experiencing read, memory, or looping errors. I am looping through a logfile and extracting the json. Before the start of the json objects there is a date and time along with message and object ID - Which corresponds to the json. So both are needed. Time is also a factor as the log file grows. I need help figuring out where I am going so wrong.

https://riptutorial.com/cplusplus/example/19029/string-streams

Multilines regex in C++

https://www.codeproject.com/Questions/1221494/Simple-multiline-regex-in-Cplusplus

http://www.cplusplus.com/reference/iterator/next/

Stringstream c++ while loop

I can use regex on a string no problem, and as a multiline. Reading from a file using stringstream I have while(input >> sstr.rdbuf()); where my stream is now buffered to my understanding.

when I cout << sstr.str() it is only read 1 time

std::ifstream input("log.txt");
std::stringstream sstr;
std::smatch m;
std::regex reg("(\\{|\\[)(\\n\\s+.*)+\\n*(\\}||\\])"); 

while (input >> sstr.rdbuf());

std::string strang = sstr.str();
while (std::regex_search(strang, m, reg)) {
    std::cout << "Results : \n" << m.str() << '\n';
    for (i = 0; i < strang.length(); i++) {
        std::cout << m.str(i);
        i++;
    }
}

This seems to loop the file forever if the file is small. For larger files 30MB+ there is no output.

I am looking at vectors and hashmaps, but I am not certain how to apply regex to hashmap- seems odd. Ontop of that I have learned that vectors only store upto about 30 variables anyway, so this type of workload is too much.

Thanks!

Another Variation

void PrintMatches(std::string str, std::regex reg) { 
    std::smatch matches;
    std::cout << matches.size() << std::endl;

}
int main() {
    std::ifstream input("log.txt");
    std::stringstream sstr;
    std::smatch m;
    std::regex reg("(\\{|\\[)(\\n\\s+.*)+\\n*(\\}||\\])");

    while (input >> sstr.rdbuf());

    std::string str = sstr.str();
    std::cout << str;
    //PrintMatches(str, reg);

    return 0;
}

Answer 1

while (input >> sstr.rdbuf()); makes no sense. operator>> on a streambuf slurps in one action, or it fails. If it fails, it almost certainly won't succeed no matter how many times you retry, and at least some of those failure modes (e.g. insertion into the output sequence failing) won't change the "truthiness" of the istream, so the loop will become infinite (which could potentially explain why you see no output on larger files, though it would be odd for slurping to fail on files that small). Take a look at an efficient (if perhaps slightly overly compact) file slurping implementation here (which will avoid at least one unnecessary copy that your code requires).

Another problem is:

strang ==sstr.str();

which is comparing an empty string to a temporary, then throwing away the result; presumably you wanted:

strang = sstr.str();

Also, this loop never runs:

for (i = 0; i > strang.length(); i++) {

By testing i > strang.length(), i is always 0 on first test, and strang will always be greater than or equal to it, so the inner loop never runs.

Your regex pattern is empty (I'm guessing omitted for brevity, but if it's really just capturing nothing, I have no idea what you were trying to do).

Finally, your while (std::regex_search(strang, m, reg)) { never changes strang, m or reg; it's either never going to run, or it will loop forever.