Doc. No.: P1221R0
Date: 2018-10-03
Reply To: Jason Rice
Title: Parametric Expressions
Audience: Evolution Working Group

Parametric Expressions


The purpose of this paper is to propose a function-like construct that creates an expression inline that can enable more idiomatic code with faster compile times.

Consider the following declaration syntax:

using add(auto a, auto b) {
  return a + b;

Here is a function-like declaration that has the same behaviour as a function without creating a function type, but the body still has its own scope.

int main() {
  int i = 40;
  int x = add(i++, 2);

// the same as

int main() {
  int i = 40;
  int x = ({
    auto&& a = i++;
    auto&& b = 2;
    a + b; // result of expression

Note that each input expression is bound to a variable resulting in evaluation that is consistent with normal function call expressions. User controlled evaluation is addressed in Using Parameters.

Function templates as we have them now are loaded with features such as overloading, constraints, type deduction, SFINAE, ect.. When called not only do we get an expensive template instantiation but also potentially large symbols and extraneous code generation all for functions we wouldn't call otherwise unless we expected the optimizer to inline them out anyways.

With this tool we want to give programmers the ability to allow this inlining at an earlier stage, but since the inlining is guaranteed and in the context of where it is called we can do things not currently possible with normal functions without creating a special type for each invocation.

This ability to transform expressions is similar to the way we use type aliases, but here we can work with constructs that have run-time effects as seen in Fusion/Hana style metaprogramming. Additionally, since this is a language feature, we could potentially see compile-time performance that is comparable to pure-type template metaprogramming.

Constexpr Parameters

Since the invocation is inlined we easily get constexpr parameters without generating a complicated symbol for a function prototype.

using to_int_c(constexpr auto x) {
  return std::integral_constant<int, x>{};

int main() {
  constexpr int x = 42;
  std::integral_constant<int, 42> foo = to_int_c(42);

Using Parameters

With using as a specifier, we allow the user to take control of evaluation with macro-like semantics.

using if_(using auto cond, using auto x, using auto y) {
  if (cond)
    return x;
    return y;

int main() {
    print("Hello, world!"),
    print("not evaluated so nothing gets printed")

This has some compelling use cases for creating idiomatic interfaces:

// customized conjunction and disjunction with short-circuit evaluation

Foo x = x || new Foo();

auto y = do_something() && maybe_do_something_else();
// constexpr ternary expression

auto x = constexpr_if(true, int{42}, std::make_unique<int>(42));
// debug logging that eliminates evaluation in production (like macros)

namespace log {
  struct null_stream {
    using operator<<(using auto) {
      return null_stream{};

  using warn() {
    if constexpr (log_level == Warning)
      return std::wcerr;
      return null_stream{};

foo > 42 && log::warn << "Foo is too large: " << foo << '\n';
              // ^ implicitly invokes nullary call

Concise Forwarding

With this feature we can implement a forwarding operator that is more concise than the existing std::forward.

using fwd(using auto x) {
  return static_cast<decltype(x)&&>(x);

auto old_f = [](auto&& x) { return std::forward<decltype(x)>(x); };
auto new_f = [](auto&& x) { return fwd(x); };

Operations on Parameter Packs

We can allow unexpanded parameter packs as inputs and avoid creating tuples in many cases.

template <typename ...Xs>
auto foo(Xs... xs) {
  auto fifth_element = pack::get<5>(xs);

  auto bar = pack::apply(f, pack::drop_front(xs));

Composing Operations

Function calls add overhead, and while they are usually inlined by the optimizer, it is still generally avoided as it can add significant compile-time overhead.

Consider the pattern of having functions return the result of other functions:

template <typename T>
auto calc_a(T input) {
  int on_the stack = input + 42;
  return calc_b(on_the_stack);

template <typename T>
auto calc_b(T& on_the_stack) {
  return calc_c(input + 1);

We can decouple these functions by using a generic continuation interface.


This is currently possible with normal functions, but it adds a bunch of noise to the call stack which makes debugging difficult, and the additional function calls make more work for the compiler which adds up.

As a Member of a Class

Parametric expressions can be used as a member in conjunction with operator overloading to create an invocable object:

  struct id_fn {
    using operator()(auto x) {
      return x;


  static_assert(std::is_invocable<id_fn, int>::value);

RAII Scope Elision

Specific conditions allow us to generate an expression that does not require RAII and therefore allows the compiler to substitute the call expression directly which can yield a prvalue and can also be used in SFINAE and other contexts where variable declarations and statements are not welcome.

using funcify(using auto f) {
  return [](auto&& ...x)
    -> decltype(f(std::forward<decltype(x)>(x)...))
    { return f(std::forward<decltype(x)>(x)...); };

struct make_array_impl {
  using operator()(using auto ...x) {
    return std::array<std::common_type<decltype(x)...>, sizeof...(x)>{x...};

constexpr auto make_array = funcify(make_array_impl{});

Comparison with Other Proposed Features

One of the primary benefits of this proposed feature is the compile-time performance and the fact that it has zero impact on the type system. That with operations on packs, there are no other proposals addressing this. However this proposed feature does have overlap with other proposed features concerning constexpr parameters and conditional evaluation.

The paper constexpr Function Parameters (P1045R1) by David Stone proposes the constexpr specifier be allowed in normal function parameter declarations effectively making the parameter constexpr. The advantage that it has over this Parametric Expressions proposal is that it allows function overloading. While this is still highly desirable, it comes with a few costs which will affect compiler throughput. First, it requires overload resolution to check if an argument expression can be used in a constant expression. Second, each constexpr parameter would affect the type creating a new function type with symbols that would have to be similar to anonymous classes for each permutation of input arguments. If this is deemed feasible, there is no conflict and the constexpr parameters proposed by this paper are consistent with David Stone's proposal and the feature proposed in this paper would be a useful augment for when overloading is not needed.

The paper Towards A (Lazy) Forwarding Mechanism for C++ (P0927R0) by James Dennet and Geoff Romer specifically sets out to add a feature to enable conditional evaluation of function parameters. The paper suggests the use of syntax similar to a lambda expression for the parameter declaration to specify laziness of evaluation which is a major shift in convention as far as variable declarations are concerned. This paper is in agreement with the motiviations stated, but the solutions are in contrast. Having the same issues with creating a function prototype, the paper addresses the implications on the type system suggesting that lazy parameters be added to the type system which is high impact, complex, and in the opinion of this paper, unnecessary. This paper, however, is proposing the use of an existing keyword in the declaration specifier which is more in keeping with existing conventions with parameter declarations, and there is no impact on the type system which makes it a better solution.

Notable Concerns

The Case for User Controlled Inlining

As far as function call inlining is concerned, optimizers bear a monopoly on this ability as far as what can be done with standard C++. Abuse of this proposed feature could lead to creating code that might be suboptimal versus having cases where the optimizer deduces that inlining should not occur.

However, there are cases where it would benefit the user to be able to prevent symbol bloat in the ABI, and while it is probably a compiler QOI issue, optimizing generic or template heavy code with -Os usually results in very large targets because the optimizer doesn't inline as agressively. Allowing the user to surgically inline functions where it makes sense can mitigate this.

Additionally inlining code on the compiler front end can bypass massive amounts of symbol generation and work for the optimizer for simple cases such as accessing the members of a large tuple. Compiler performance is no small issue and many go to great lengths to avoid libraries that, while providing interfaces that are more idiomatic and make applications more maintainable, they add significant overhead to build times effectively killing productivity. Adding parametric expressions would solve this.

Why not go for broke and jump straight into granular AST node manipulation?

Some have suggested that we add something more akin to raw AST generation as is found in Rust macros. While this would be a powerful superset of the feature this paper is proposing, it comes with some issues. One, this would make the interface significantly more complicated and inaccessible to typical users. It also comes with the same problems with evaluation that preprocessor macros have in that it easily allows mistakes that result in duplicate evaluation or double move operations.

The interface proposed with Parametric Expressions is such that it defaults to consistent evaluation of input expressions by binding them to an actual variable declaration in the parameter list. If the author of a function wants to take control of evaluation they can do so via the using parameter declaration specifier and thus opt in explictly and without a lot of crazy AST syntax.

That said, the advent of Reflection and Metaclasses may open the door to allowing more refined control over the AST, and this paper's stance is that this feature is forwards compatible with the possibilty of adding reflexpr parameters or some similar construct to enable this superset.

Invocable and the Dangers of using Parameters

As previously stated, using parameters add macro-like semantics. While the pitfalls of macros are well understood, here is a particularly onerous edge case that is worth mentioning:

struct eval_twice_fn {
  using operator()(using auto x) {
    return std::pair(x, x);
} inline constexpr eval_twice{};

auto result1 = eval_twice(my::string("foo"));
auto result2 = std::invoke(eval_twice, my::string("foo"));

Here the semantics are changed when used with std::invoke. Since std::invoke is a normal function, it evaluates it and binds it to a parameter that is perfectly forwarded. The forwarding expression would then be passed to eval_twice and the result of the input would get moved twice. With the call expression the result is a pair of two distinct strings containing a pointer to their own "foo".

std::Invokable does not require that the call to std::invoke be equality preserving on its arguments so this is likely permissible. Regardless, this paper's stance is to allow these types of edge cases, and leave it to the discretion of the user as the user is opting in explicitly via the using specifier. Attempts to prevent these would involve complex rules and could inadvertantly rule out desirable use cases.

Impact on the Existing Standard

While there is zero impact on existing features in C++, the following additions are required:

  1. Parsing the using keyword followed by an identifier or operator name followed by a left parentheses would indicate the declaration of a parametric expression disambiguating from other valid uses of the using keyword for other types of declarations.

  2. Allow the constexpr specifier in parameter declarations in the context of a parametric expression parameter list declaration.

  3. The using keyword is used as a declaration specifier for parameter declarations in the context a parametric expression parameter list declaration.

  4. The auto keyword is used as placeholder for a constraint for parametric expression parameters and it implictly promotes parameters as auto&&. This is more concise but a slight deviation from what C++ programmers might expect.

The Rules

The following rules have been refined based on experience from the reference implementation:

  1. Declaration or invocation does not create a type. Overloading or ADL is not supported.

  2. The input parameter list may contain only one parameter pack in any position.

  3. Input parameter type specifiers must be completely unconstrained and not specify a type.

    • Only auto is allowed as the type specifier. The idea here is to be forwards compatible with a concise syntax for specifying a constraint via the concepts language feature.
    • The other option is to allow specifying a type, but this is ugly and not useful as overloading is not supported.
  4. Parameter declarations can have constexpr or using specifiers which change semantics:

    • With none of these specifiers the parameters is an rvalue reference initialized with the input expression. Its name is treated as an lvalue. (just like auto&&)
    • The constexpr specifier is a value initialized with an expression that must be a constant-expression
    • The using specifier does not create a variable and the input expression is substituted anywhere the name is used. (like a macro)
  5. The input parameters may be unexpanded parameter packs.

    • This adds a bit more power for working with lists.
  6. The definition is a compound statement that may have RAII scope when invoked.

    • A RAII scope is not created if there is only one statement that is a return statement, and all parameters (if any) are declared with the using specifier.
    • A RAII scope is created if there are any statements other than a single return statement or if there are any variable declarations without the using specifier.
    • Implicit this does not count as a variable declaration for the purposes of detecting if there should be a RAII scope.
  7. If a RAII scope is created then invocation executes the compound statement as an expression with the result being determined by its return statements with the same RVO/NRVO guaranteed copy elision rules that apply to normal functions.

  8. If no RAII scope is created then invocation results in substituting the expression from the return statement.

  9. As with deduced return types in functions, the return statements must all result in the same type not considering statements omitted after static selection.

  10. Recursion is not allowed

    • Recursion is not feasible, but we could at least use Odin Holmes' recursive alias pattern
  11. Operator overloading is supported. (only as a member since ADL is not supported)

  12. When defined as a member of a class, invocations have implicit this. An optional const qualifier after the parameter list can be used restrict this to const.

    • This is consistent with the way member functions behave.
    • An object with a defined using operator() should be Invocable.
  13. As a member, the static keyword can be used to simply make it a free entity that is local to the owning class.

    • This is consistent with the way member functions behave.
  14. The use of an identifier that refers the name of a parametric expression, when used in an expression without a call expression (ie no parens), is implicitly promoted to a nullary call.

    • This is a nice-to-have as it can reduce syntatic noise and make nullary parametric expressions appear as a variable.


With this we have a facility for writing more idiomatic code and with a significant improvement in compile-time performance having an impact that would rival what was seen with the advent of variadic templates.

This feature has the potential to have a huge impact on the ability to implement ranges, expression templates, metaprogramming libraries, and functional and generic code as well as replace the use of macros in a lot of use cases. It would be really cool to get this in C++20 if it is not too late.

Thanks for looking at this!

Jason Rice


Special thanks to Louis Dionne and Miguel Ojeda who provided feedback and suggestions as well as others who provided feedback on the mailing list.

Parametric Expressions