Class to calculate performance of a function - c++

Question

I'm writing a class as follows:
struct TimeIt {
using TimePoint = std::chrono::time_point<std::chrono::high_resolution_clock>;
TimeIt(const std::string& functName) :
t1{std::chrono::high_resolution_clock::now()},
functName{functName} {}
~TimeIt() {
TimePoint t2 = std::chrono::high_resolution_clock::now();
std::cout << "Exiting from " << functName << "...\n Elapsed: ";
std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1).count() << " ms" << "\n";
}
TimePoint t1;
std::string functName;
};
The whole point of it is measure the time that takes for one function to complete, calling this at the start of it. However, the only value I'm getting is 0ms. This is obviously wrong, because it takes up to a minute for some of the functions, but I can't see why it's wrong.
I did the same, but at the start and end of the function, creating the TimePoint (with auto) and doing a duration_cast. Any clue what I'm missing here?
Edit:
I'm going to try to make it reproducible. A little bit of context: I'm working with big matrixes (12000 dimensions) and doing a lot of input output operations.
template <typename InputType>
InputMat<InputType>
readInp(const std::string& filepath = "data.inp", const size_t& reserveSize = 15000) {
TimeIt("readInp");
std::ifstream F(filepath);
assert(F.is_open());
InputMat<InputType> res;
res.reserve(reserveSize);
std::string line;
while (F >> line) {
InputType lineBitset{line};
res.push_back(lineBitset);
}
return res;
}
This function reads a matrix, and calling TimeIt here gives really different results compared when I call it in the wrapper function:
void test1() {
//Testing for 0-1 values
auto t1 = std::chrono::high_resolution_clock::now();
auto inpMat = readInp<std::bitset<32>>();
auto t2 = std::chrono::high_resolution_clock::now();
std::cout << std::chrono::duration_cast<std::chrono::milliseconds>(t2 - t1).count() << "\n";
//More code...
}
This outputs:
Exiting from readInp...
Elapsed: 0 milliseconds
4
and data.inp

Answer 1

NOW you have made the problem clear! By writing this:
TimeIt("Reading");
you are creating a temporary object, which is immediately deleted. You need to give this object a name so it lives until the end of the block:
TimeIt timer("Reading");