如何使用C 20的STD :: source_location以获得更好的记录？

使用std::source_location::current() 作为默认参数可自动捕获调用点的文件名、行号和函数名；2. 可通过宏如#define LOG(msg) log(msg, std::source_location::current()) 简化日志调用；3. 可结合日志级别、时间戳等信息扩展日志内容；4. 为优化性能，可省略函数名或在发布版本中禁用位置信息；5. column() 等细节较少使用，但可用。使用std::source_location 能以极低开销显着提升日志的调试价值，无需手动传入FILE 或__LINE__，让编译器自动完成上下文捕获，从而实现更高效、清晰的诊断输出。

std::source_location in C 20 is a lightweight utility that captures information about the source code's context—like file name, line number, function name, and column—at the point where it's used. It's incredibly useful for improving logging by automatically including diagnostic context without requiring manual input.

Here's how to use std::source_location effectively for better logging.

1. Include and Capture Source Location Automatically

std::source_location is defined in the <source_location></source_location> header (C 20). It's a constexpr -friendly class that captures location info at the call site when used with default arguments.

 #include <iostream>
#include <string_view>
#include <source_location>

void log(std::string_view message,
         std::source_location location = std::source_location::current())
{
    std::cout << location.file_name() << ":"
              << location.line() << " "
              << location.function_name() << "() - "
              << message << &#39;\n&#39;;
}

Now when you call log("Something happened"); , the file, line, and function are captured automatically.

Example usage:

 void my_function() {
    log("Debug message"); // Automatically prints file, line, function
}

Output might look like:

 main.cpp:15 my_function() - Debug message

2. Use in Macros for Cleaner Logging

To avoid passing source_location manually every time, wrap the logging in a macro—though with C 20, you often don't need macros thanks to default parameter capture.

But if you want a clean interface (eg, LOG("msg") ), a macro helps:

 #define LOG(msg) log(msg, std::source_location::current())

// Usage
LOG("Error occurred");

This ensures the location is captured at the macro call site, not inside the log function.

Note: Since std::source_location::current() is a built-in compiler intrinsic, the macro isn't strictly necessary, but it makes the syntax cleaner and more familiar.

3. Customize Log Format and Add Extra Context

You can extend the logger to include timestamps, log levels, or custom tags.

 enum class LogLevel {
    Debug,
    Info,
    Warning,
    Error
};

void log(LogLevel level, std::string_view message,
         std::source_location loc = std::source_location::current())
{
    const char* level_str[] = {"DEBUG", "INFO", "WARNING", "ERROR"};

    std::cout << "[" << level_str[static_cast<int>(level)] << "] "
              << loc.file_name() << ":" << loc.line() << " "
              << loc.function_name() << " - "
              << message << &#39;\n&#39;;
}

Usage:

 log(LogLevel::Warning, "Value out of range");

Output:

 [WARNING] config.cpp:42 parse_config() - Value out of range

4. Optimize for Performance

std::source_location is lightweight, but capturing function names (which may include full signatures and templates) can be expensive in terms of binary size or performance.

If you don't need the function name, skip it:

 std::cout << loc.file_name() << ":" << loc.line();

Or, use conditional compilation to disable location info in release builds:

 #ifdef DEBUG
    auto loc = std::source_location::current();
    std::cout << loc.file_name() << ":" << loc.line() << " ";
#else
    std::cout << "[debug info disabled] ";
#endif

Alternatively, design your logging system so that debug-level logs (with full location) are compiled out when not needed.

5. Capture Column and Other Details (Rarely Used)

std::source_location also provides column() and file_name() / function_name() as const char* . Column is rarely useful but available.

 std::cout << "Column: " << loc.column() << &#39;\n&#39;; // Usually 1 or not very meaningful

Most compilers set column to the start of the statement, so it's not commonly used.

Summary

• Use std::source_location::current() as a default function parameter to auto-capture location.
• Avoid macros if you're okay with plain function calls; otherwise, use them for cleaner syntax.
• Include file, line, and function in logs to speed up debugging.
• Be mindful of performance and binary bloat from long function names.
• Combine with log levels and formatting for production-ready logging.

Using std::source_location makes logging more informative with almost zero effort—no more manually writing __FILE__ or __LINE__ . It's a small feature with a big impact on debuggability.

Basically, just add it to your logging function and let the compiler do the rest.

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