C++ Variable I/O

Question

Developing a C++ program and have a requirement of outputting several float variables to a file, to later read them back into another program. What I/O statements would be used to do this? Sid Kraft

Answer 1

For writing and later reading float values between C++ programs, use standard file I/O streams. You can choose text or binary format depending on your needs.

Text-mode I/O (human-readable)

#include <fstream>

void write_floats_text(const char* filename, float a, float b, float c)
{
    std::ofstream out(filename);          // text mode by default
    out << a << ' ' << b << ' ' << c;     // formatted text output
}

void read_floats_text(const char* filename, float& a, float& b, float& c)
{
    std::ifstream in(filename);
    in >> a >> b >> c;                    // formatted text input
}

This uses ofstream/ifstream and the stream insertion (<<) and extraction (>>) operators. It is portable and easy to inspect, but may lose some precision depending on formatting.

Binary-mode I/O (exact bit pattern)

#include <fstream>

void write_floats_binary(const char* filename, float a, float b, float c)
{
    std::ofstream out(filename, std::ios::binary);
    out.write(reinterpret_cast<const char*>(&a), sizeof(a));
    out.write(reinterpret_cast<const char*>(&b), sizeof(b));
    out.write(reinterpret_cast<const char*>(&c), sizeof(c));
}

void read_floats_binary(const char* filename, float& a, float& b, float& c)
{
    std::ifstream in(filename, std::ios::binary);
    in.read(reinterpret_cast<char*>(&a), sizeof(a));
    in.read(reinterpret_cast<char*>(&b), sizeof(b));
    in.read(reinterpret_cast<char*>(&c), sizeof(c));
}

This uses ofstream/ifstream in binary mode and the write/read member functions. It preserves the exact float representation, but the file is not human-readable and both programs must agree on layout and endianness.

For simple cross-program exchange on the same platform, either approach is valid. Use text mode for readability and easier debugging; use binary mode when exact bit-level fidelity and performance are more important.

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References:

• Output Streams
• Output File Stream Member Functions
• Input/Output Streams
• Input and output