[SOLVED] How to pass data from a std::ostringstream to a function parameter const char *data without copying the data (if possible)

How to pass data from a std::ostringstream to a function parameter const char *data without copying the data (if possible)

In existing code I have a large block of data (around 200KB) constructed in a std::ostringstream payload;

I need to pass this to an API function as const char* data:

int API_function(const char* data, int len)

I am currently using:

API_function(payload.str().c_str(), payload.str().length())

But if the payload has bytes with value = 0x00 the char * will be truncated.

How can I convert my payload to char* data without copying it (if possible) as this is running on a constrained micro-controller ?

Solution

But if the payload has bytes with value = 0x00 the char * will be truncated.

Not true, most probably you are confused by debugger. Length should extend beyond this zero (see example at bottom), when debugger displays const char* it stops displaying data when encountering zero, but data are there.

How can I convert my payload to char* data without copying it (if possible) as this is running on a constrained micro-controller

Since C++20 you can use std::ostringstream::view().

#include <print>
#include <sstream>
#include <string_view>

using namespace std::literals;

int API_function(const char* data, int len)
{
    std::string_view v{data, static_cast<size_t>(len)};
    std::println("  Wrong use of data: {}", data);
    std::println("Correct use of data: {}", v);
    
    return len;
}

int main()
{
    std::ostringstream out;
    out << "Some string\0with zero inside. "sv << 13;
    API_function(out.view().data(), out.view().size());
}

Live demo.