Modern C++ Programming Series: Variadic Function Templates

Introduction

• Lesson on variadic function templates in C++
• Function templates that can take an arbitrary number of parameters
• Popular in C programming (e.g., printf function)

Background

• Variadic functions in C:
  • Use of stdarg.h (or cstdarg in C++) for variable arguments
  • Involves setting up a structure to handle arguments and determine types
  • Disadvantage: runtime cost due to argument detection

Variadic Function Templates in C++

• Use templates to generate functions for any number of arguments
• Benefits:
  • No runtime checks needed
  • Exact function generated for required operations

Example: Sum Function

• Goal: Create a function sum to add an arbitrary number of numbers
• Initial simple function for one argument: int sum(int arg) { return arg; }

Converting to Template

• Transform to handle multiple types: template <typename T> T sum(T arg) { return arg; }
• Handle multiple arguments using parameter pack: template <typename T, typename... Args> T sum(T first, Args... args) { return first + sum(args...); }

Parameter Packs

• Syntax: typename... Args, Args... args
• Allows collection of an arbitrary number of arguments

Handling Different Types

• Issue with mixed types:
  • Example: sum(1, 2.2, 3.7, 4, 5) results in incorrect total
• Insight: Template instantiation can cause type issues
• Solution: Ensure first two numbers are handled as double for accuracy

Code Size Consideration

• Variadic templates generate multiple instantiations
• Important to consider in domains sensitive to code size

Practical Use

• Useful for creating functions like custom print or sum functions
• Test thoroughly to ensure type correctness

Conclusion

• Variadic templates offer flexibility and efficiency over traditional C variadic functions
• Always test and validate code
• Encouraged to explore more with concepts like constexpr for compile-time evaluation

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