How can I replace the C-style string parsing with C++ core guideline-compliant code?

Question

Take for example these two code fragments:

//...
file_string = strstr(file_string, "\nv ");
while (file_string = strstr(file_string, "v ")) {
    vec::vec3<float> buffer = { 0.0f };
    file_string += strlen("v ");
    file_string = std::from_chars(file_string, file_string_end, buffer.x).ptr;
    file_string++;
    file_string = std::from_chars(file_string, file_string_end, buffer.y).ptr;
    file_string++;
    file_string = std::from_chars(file_string, file_string_end, buffer.z).ptr;
    file_string++;
    vcoords.push_back(std::move(buffer));
}
//...

//...
while (file_string = strstr(file_string, "v ")) {
    size++;
    file_string++;
}
vcoords.reserve(size);
//...

For this type of data

(...)
v 1.000000 1.000000 -1.000000
v 1.000000 -1.000000 -1.000000
v 1.000000 1.000000 1.000000
v 1.000000 -1.000000 1.000000
v -1.000000 1.000000 -1.000000
(...)

they work and they work sufficiently quickly. They also generate warnings such as:

C26481: Don't use pointer arithmetic.

C26486: Don't pass a pointer that may be invalid to a function. Parameter 0 'file_string' in call to 'strstr' may be invalid (lifetime.3).

How can I replace the strstr/pointer arithmetic mix with something that would do its job comparably fast and not generate such warnings? I tried going through similar questions related to std::string-to-float conversions but they either used std::stringstream, which is very slow, or assumed that the string in question contained nothing but one value.

Answer 1

edited for changes after enabling core guidelines checker

I took a crack at using std::from_chars. Fun fact: it doesn't work on gcc or clang for floating point values yet! That... might warrant not using it until the feature is mature.

Things to shut up the guidelines checker:

• gsl::at is the cheat code for pointer arithmetic. No warning
• The lifetime checker is kind of dumb. It doesn't know a sting literal wrapped by a string_view still has infinite lifetime, but it doesn't flag if you use string view literals (""sv).
• Still using string::find for searching, then adding the position to string::data()
• I recommended indexing instead of pointer arithmetic before, but the analysis doesn't like that either.

using namespace std::literals;
const std::string file_string = "\n"
    "v 1.000000 1.000000 -1.000000\n"
    "v 1.000000 -1.000000 -1.000000\n"
    "v 1.000000 1.000000 1.000000\n";
const auto tag = "v "sv;
const char* file_string_end = &gsl::at(file_string, file_string.size());

std::vector<vec::vec3<float>> vcoords;
std::string::size_type pos = 0;
while ((pos = file_string.find(tag, pos)) != std::string::npos) {
    vec::vec3<float> buffer = { 0.0f };
    auto [x_ptr, x_ec] = std::from_chars(
        &gsl::at(file_string, pos + tag.size()), 
        file_string_end, 
        buffer.x);
    if (x_ec != std::errc()) {
        throw std::runtime_error("bad x");
    }
    std::string_view x_view(x_ptr);
    auto [y_ptr, y_ec] = std::from_chars(
        &gsl::at(x_view, 1), 
        file_string_end, 
        buffer.y);
    if (y_ec != std::errc()) {
        throw std::runtime_error("bad y");
    }
    std::string_view y_view(y_ptr);
    auto [z_ptr, z_ec] = std::from_chars(
        &gsl::at(y_view, 1), 
        file_string_end, 
        buffer.z);
    if (z_ec != std::errc()) {
        throw std::runtime_error("bad z");
    }
    vcoords.push_back(buffer);
}

https://godbolt.org/z/F9j7FA