Modern C++: Variadic Templates

This follows my series to cover lesser known features of Modern C++

Today, we have variadic templates, which is an improvement over variadic arguments (...), a type of argument that can take variable arguments.

Before we start with variadic templates, an introduction to variadic arguments is required.

Variadic Arguments

Variadic arguments have been around since the time of C and old C++ but it has a lot of limitations, which can be seen in the code demonstration below:

#include <iostream>
#include <cstdarg>

void printNumbers(int count, ...) {
    va_list args; // Hold and make argument list with capacity "count"
    va_start(args, count); // Start processing variable args

    for (int i = 0; i < count; ++i) {
        int num = va_arg(args, int); // Get next argument
        std::cout << num << " ";
    }

    va_end(args); // Clean up
    std::cout << std::endl;
}

int main() {
    printNumbers(1, 2, 3, 4); // Works completely fine
    printNumbers(4, 34, '4', 12.3, "12"); // It doesn't raise error but IT SHOULD!
    return 0;
}

As can be seen above:

• Number of arguments need to be provided manually (most of the time, it's dynamic)
• The compiler doesn't raise any error if variable arguments have different types
• Need to import header file <cstdarg> to use them
• It only works with basic primitive types (std::string is NOT)

This is very limited and doesn't work for the Modern C++ world where classes, objects and templates are used all the time.

Variadic Templates

Variadic Templates were introduced in C++ 11 to eliminate the limitations of variadic arguments by providing type safety, dynamic argument length and lesser usage of macros.

The following code is a basic example of variadic templates used to sum numbers:

#include <iostream>

template<typename... Args>
auto sum(Args... args) {
    return (args + ...); // Fold expression (C++17)
}

int main() {
    std::cout << sum(1, 2, 3, 4, 5) << std::endl; // Output: 15
    std::cout << sum(2.5, 3.5) << std::endl;      // Output: 6.0
}

The function sum has a packet parameter Args... args which can accept any number of arguments

(args + ...) is a fold expression, introduced in C++ 17, and is a clean and concise way to perform operations over variadic templates.

This code is then expanded to ((1 + 2) + 3) + 4 + 5, which returns the value of 15.

More than Functions

Variadic Templates have more usage than just using them with basic functions. They can be used in situations like:

1. Logical AND / OR Boolean Operations

#include <iostream>

template<typename... Bools>
bool allTrue(Bools... bs) {
    return (bs && ...);
}

template<typename... Bools>
bool anyTrue(Bools... bs) {
    return (bs || ...);
}

int main() {
    std::cout << allTrue(true, true, false) << "\n"; // false
    std::cout << anyTrue(false, false, true) << "\n"; // true
}

This operation is used in flag evaluation and validation checks

1. Calling a Function on Each Argument

#include <iostream>

void log(int x) { std::cout << "[int] " << x << "\n"; }
void log(const char* s) { std::cout << "[str] " << s << "\n"; }

template<typename... Args>
void logAll(Args... args) {
    (log(args), ...); // C++17 fold expression with comma
}

int main() {
    logAll(1, "hello", 42, "world");
}

The code snippet logs all the arguments according to their type.

[int] 1
[str] hello
[int] 42
[str] world

1. Custom Factory functions

#include <memory>
#include <utility>
#include <iostream>

template<typename T, typename... Args>
std::unique_ptr<T> make_instance(Args&&... args) {
    return std::make_unique<T>(std::forward<Args>(args)...); // preserve the argument was originally passed
}


struct Foo {
    Foo(int x, const std::string& y) {
        std::cout << "Foo constructed: " << x << ", " << y << "\n"
    }
};



int main() {
    auto f = make_instance<Foo>(10, "bar");
}

The code snippet is a custom factory function which makes a class with any number of parameters.

Foo constructed: 10, bar

1. Real life applications

Variadic templates are heavily used by C++ STL such as std::tuple, std::pair, std::array std::make_unique, std::make_shared libraries

When to use C variadic arguments va_arg

• Working with C functions such as printf, scanf, syslog, etc
• Extending C code on top of C++ code
• Writing bindings for legacy support

Variadic Arguments to Variadic Templates

Variadic Arguments can easily be wrapped with Variadic Templates incase there's a need to do it:

#include <cstdarg>
#include <cstdio>
#include <string>

void c_log(const char* fmt, ...) {
    va_list args;
    va_start(args, fmt);
    vprintf(fmt, args);
    va_end(args);
}

template<typename... Args>
void logSafe(const std::string& fmt, Args... args) {
    // c_str() method turns std::string into char*
    c_log(fmt.c_str(), args...);
}

int main() {
    logSafe("Number: %d, Text: %s\n", 42, "Hello");
}

This example demonstrates how a C-style variadic, which logs a message, is wrapped by a Variadic Template.

Conclusion

• Use C variadic functions only when you need to interop with legacy C code or third-party libraries.
• For all modern C++ code, prefer variadic templates for type safety, maintainability, and cleaner syntax.
• You can even wrap C variadic functions with variadic templates when needed.

Code

All the code used in this article is available here.

Delta456 / modern_cpp_series

Code for my Modern C++ Articles

Modern C++ Series

Code for my Modern C++ Article Series.

Articles

• Modern C++: An Introduction
• Modern C++: Raw strings R("string")
• Modern C++: and, or and not as bool operators
• Modern C++: constexpr
• Modern C++: Temp Vars in if/switch statments
• Modern C++: Lambdas
• Modern C++: enum class|struct
• Modern C++: std::optional

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