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Coding Patterns - strings

Coding Patterns - strings
- Simulating sprintf
- Raw character reading

Simulating sprintf

Using a C-string printf() is convenient for formatting, but troublesome for memory management and avoidance of buffer overflows. And mostly the resulting output will be needed as a C++ string as well.

The solution is to combine C and C++ strings and use snprintf() to limit the possibilty of overflow:

//  some type of printf...
{
    std::string output;

    // Use snprintf with zero to establish the size (without '\0') required
    auto n = ::snprintf(nullptr, 0, fmt, val);

    if (n > 0)
    {
        output.resize(n+1);
        char* buf = &(output[0]);

        // Format directly into buffer
        n = ::snprintf(buf, n+1, fmt, val);
        output.resize(n);
    }
    else
    {
        output.clear();
    }

    return output;
}

The intermediate buf variable is not required, but makes it more apparent what is happening.

Raw character reading

It is sometimes necessary to read number of raw characters from a C++ stream, or a C FILE directly. For this we also use the memory management and encapsulation of std::string:

std::string my_get(std::istream& is, std::size_t len)
{
    std::string output;
    output.resize(len);
    char* buf = &(output[0]);

    is.read(buf, len);

    return output;
}

The intermediate buf variable is not required, but makes it more apparent what is happening.

Anti-pattern: Using a variable length buffer. The C++ specification allows static allocation, but its dimension must be constexpr. The GCC compiler will not complain; CLANG will - it is non-conforming.

{
    char buf[len];      // This only works if len is constexpr!!

    is.read(buf, len);

    return buf;         // Auto-convert to std::string
}