std::source_location in Modern C++: A Better Way to Build Debugging, Logging, and Diagnostics Systems

In low-level and high-performance C++ systems, one of the most persistent challenges is obtaining accurate, maintainable, and low-overhead diagnostic information.

For decades, developers relied on macros such as:

__FILE__
__LINE__
__FUNCTION__

While effective, these approaches suffer from serious limitations:

• They rely on macros (not type-safe)

• They pollute interfaces

• They are difficult to integrate into modern abstractions

• They lack composability

With C++20, the language introduces a powerful and elegant solution:

std::source_location — a zero-overhead, type-safe mechanism for capturing call-site information

What is std::source_location?

std::source_location is a lightweight object that captures:

• File name

• Line number

• Column number

• Function name

It is defined in:

#include <source_location>

The key capability lies in:

std::source_location::current()

This function retrieves information about the call site, not where it is written.

Basic Example

#include <iostream>
#include <source_location>
void log(const std::source_location& loc = std::source_location::current()) {
    std::cout << "File: " << loc.file_name() << "\n"
              << "Line: " << loc.line() << "\n"
              << "Function: " << loc.function_name() << "\n";
}
int main() {
    log();
}

Output (example)

File: main.cpp
Line: 12
Function: main

Why std::source_location is Important

1. Eliminates Macros

Old approach:

#define LOG(msg) log(msg, __FILE__, __LINE__)

Modern approach:

void log(std::string_view msg,
         const std::source_location& loc = std::source_location::current());

No macros. Cleaner. Safer.

2. Call-Site Transparency

The most powerful feature is:

The default argument is evaluated at the call site, not at the function definition.

This enables writing reusable utilities without losing context.

3. Zero Runtime Overhead

• No heap allocation

• Typically optimized as compile-time constants

• Comparable cost to macros

This makes it suitable even for:

• real-time systems

• embedded systems

• high-frequency logging

Practical Usage Patterns

1. Building a Modern Logging System

#include <iostream>
#include <source_location>
void log_info(std::string_view message,
              const std::source_location& loc = std::source_location::current()) {
    std::cout << "[INFO] "
              << loc.file_name() << ":" << loc.line()
              << " (" << loc.function_name() << ") "
              << message << "\n";
}

Usage:

log_info("Server started");

2. Error Handling Without Macros

#include <stdexcept>
#include <source_location>
[[noreturn]]
void fail(std::string_view msg,
          const std::source_location& loc = std::source_location::current()) {
    throw std::runtime_error(
        std::string(msg) + " at " +
        loc.file_name() + ":" +
        std::to_string(loc.line()));
}

3. Assertions System

#include <iostream>
#include <cstdlib>
void assert_true(bool condition,
                 const std::source_location& loc = std::source_location::current()) {
    if (!condition) {
        std::cerr << "Assertion failed at "
                  << loc.file_name() << ":" << loc.line() << "\n";
        std::abort();
    }
}

4. Integration with High-Performance Backends

In systems like:

• Boost.Asio servers

• trading engines

• distributed systems

You can attach source_location to:

• log entries

• tracing events

• metrics systems

without affecting performance.

Advanced Usage (Important for Experts)

Passing Explicit Source Location

Sometimes you want to propagate location manually:

void wrapper(const std::source_location& loc = std::source_location::current()) {
    log_info("Wrapped call", loc);
}

Storing source_location

You can embed it into structures:

struct ErrorContext {
    std::string message;
    std::source_location location;
};

Combining with std::expected

#include <expected>
std::expected<int, ErrorContext> parse() {
    return std::unexpected(ErrorContext{
        "parse error",
        std::source_location::current()
    });
}

This creates rich error propagation without exceptions.

Best Practices for Efficient Usage

1. Always Use Default Parameter Pattern

const std::source_location& loc = std::source_location::current()

This ensures:

• correct call-site capture

• zero additional effort for callers

2. Avoid Manual Passing Unless Necessary

Only pass explicitly when forwarding context.

3. Do Not Overuse in Tight Loops

Even though it is lightweight:

• avoid excessive logging in hot paths

• use conditional logging

4. Combine with Compile-Time Logging

Use with:

• constexpr flags

• compile-time log levels

to eliminate overhead entirely in release builds.

5. Replace Legacy Macros Gradually

You can migrate:

From:

#define LOG_ERROR(msg) ...

To:

inline void log_error(std::string_view msg,
    const std::source_location& loc = std::source_location::current());

Comparison with Other Languages

• Rust: uses file!(), line!() macros

• C++: now provides a type-safe alternative without macros

This aligns C++ with modern language design principles while preserving performance.

Final Insight

std::source_location is not just a small utility feature.

It represents a design shift in Modern C++:

Moving from macro-based debugging to type-safe, composable diagnostics.

For advanced developers building:

• compilers

• backends

• system tools

• performance-critical applications

this feature becomes a foundational building block.

Closing Thought

The difference between basic logging and professional-grade diagnostics systems often lies in details like this.

And std::source_location is one of those details that quietly transforms your entire architecture.