1. Problem It Solves
Callable code must handle free functions, member pointers, and function objects consistently. std::invoke_r also requests a result type, while std::bind_back stores arguments at the end of a future call. The goal is to make the relevant rule visible in code instead of relying on an assumption about what “C++23 support” means.
2. Prerequisites
Day 28: type-erased callables.
Day 29: forwarding and argument categories.
3. Core Idea
std::invoke is the universal call button. The _r form places a result-type adapter around it; bind_back creates a new button with the last slots already filled. Read the syntax from left to right, identify the value, object, or range being transformed, and then check its resulting type, lifetime, or ownership. Standardization and implementation are separate: a C++23 mode may still lack one library component, so a feature-test macro is part of responsible portable use.
4. Minimal Syntax
long n = std::invoke_r<long>(callable, 2, 3);5. How It Works
The sample builds the smallest expression or object needed for
std::invoke_r,std::bind_back, and Callable Utilities.It invokes addition with an explicit result type and conditionally binds a trailing argument. The compiler and library apply the relevant rule before the program observes the result.
The program prints or checks a converted sum and, when available, the result of the bound callable, making the important behavior easy to verify.
6. Common Mistakes
A requested
Rthat is not implicitly convertible from the actual result makesinvoke_rill-formed; storing references in a bound object can outlive their referents.Enabling C++23 mode without checking the relevant feature macro may select code that the installed compiler or standard library does not implement yet.
7. When to Use It
Use it when the task involves generic dispatch layers and small partial applications with clear ownership of bound values.
Avoid it when the task involves complex argument rearrangement where a named lambda is easier to read.
8. Simple Example
A filter factory binds a fixed upper limit at the back and leaves the measured value as the future argument. The downloadable program keeps the data fixed so the output can be compared without entering input.
Complete sample code
Source file
cpp23/30_invoke_r_bind_back_callables/main.cpp
#include <functional>
#include <iostream>
#include <version>
int main() {
#if defined(__cpp_lib_invoke_r)
long result = std::invoke_r<long>(std::plus<>{}, 2, 3);
#else
long result = std::plus<>{}(2, 3);
#endif
std::cout << "invoke_r=" << result << '\n';
#if defined(__cpp_lib_bind_back)
auto add_ten = std::bind_back(std::plus<>{}, 10);
std::cout << "bind_back=" << add_ten(5) << '\n';
#else
std::cout << "bind_back unavailable\n";
#endif
}
9. Key Takeaways
Separate the language rule from compiler and library availability.
Keep lifetime, ownership, and deduced types visible when they affect correctness.
Prefer the smallest syntax that communicates the intent.
Guard facilities that are not yet uniformly implemented.
10. Self-Check Questions
Easy — What is the smallest syntax in Section 4, and what main job does it perform?
Medium — Read the sample program: which value, type, or branch is observed, and why?
Hard — If a callable returns
double, when isstd::invoke_r<int>valid, and how does this differ from requiring the callable to declareintitself?