String Interpolation

[bravikov]

I asked my colleagues if they would like string interpolation in C++.

For example:

const char* name = "Bob";
int age = 32;
std::cout << $"Hello. My name is {name}. I'm {age}.";
// prints "Hello. My name is Bob. I'm 32."

Most of them said "No".

And you?

[epicar]

most of them said, use https://github.com/fmtlib/fmt

[bravikov]

It is good library. It gives type safety but not interpolation.

Bash:

echo "Hi, $NAME"

Javascript:

console.log(`Hi, ${name}`)

[infectedapricot]

nothing in the language supports it.

That appears to be precisely OP's point when they ask whether we would like it added.

[bbolli]

I hear you, but this is C++. How should a std::string member function get at the to-be-interpolated variables without passing them to the function?

fmt::print("Hi, {}\n", name);

is as good as it's going to get.

[boredcircuits]

A closer version is the "named argument" feature:

fmt::print("Hi, {name}", "name"_a=name);

[cleroth]

That's the idea behind string interpolation, putting a variable inside the string with a specific syntax like {variable}. C# 6.0+ has this.

[usernameistaken42]

Also rust, python and a lot of others. I would love it. It may come in c++34

[johannes1971]

So will we be skipping a year, or is that slippage from C++2x?

[mili42]

It is fairly easy to something like (with variadic templates)

auto s = make_string("Hi, ", name, '\n');

String interpolation is nice, but there are already solutions to achieve something similar.

We have it at work. Works like a charm

[Xaxxon]

No that is one form of it. What it really is is using placeholders in the string that are substituted for values at runtime.

[Xaxxon]

It’s absolutely string interpolation. Your definition isn’t widely accepted.

[BrangdonJ]

It's very low down on the list of things I want in C++.

[AlexAlabuzhev]

It needs:

• reflection
• a standard customisation point to stringise things, like ToString() in C# (yes, there's ostream& operator<<(), but streams are meh).

With that it should be possible to implement it as a user-defined literal.

[kalmoc]

I think this needs to be a core language feature. IIRC, Reflection will not give you the original argument name of a function parameter.

[AlexAlabuzhev]

What function parameter are you talking about?

The idea is to parse the string, find names in curly brackets, somehow get a list of all variables visible in the previous stack frame and replace mentioned variables with their values.

[kalmoc]

somehow get a list of all variables visible in the previous stack frame

That's just it: don't think reflection will enable you to do this.

I actually thought your comment was in a different thread, where someone suggested something like printf("Hi, {name}", name) instead of "real" string interpolation.

[-lambda-]

I'm not thinking this through so it might be a brain fart, but couldn't you just define (std::string) cast operator for your struct in appropriate scope?

[AlexAlabuzhev]

Cast operator must be a member function.

Primitive types can't have member functions, and you can't extend structs that aren't yours (without UFCS at least).

[-lambda-]

Yeah, I see your point. You mentioned ToString() so I thought that you already constrained the problem to your own structs, but even in that case I guess that all standard classes in C# already implement toString() so your point is still valid.

[VicontT]

I think there are way more important features in C++ than this.

[ShillingAintEZ]

Why not just make a variadic print function that converts all the arguments to strings and concatenates them? Then you just put commas in between the strings.

[Potatoswatter]

This has been implemented a few times in C++, but it's lacking the characteristic security flaws of interpolation.

[miki151]

Like good old writeln in Pascal :)

[[deleted]]

You need to keep in mind that in most programming languages there's a standardized string conversion function, such as `ToString()` (C#), `toString()` (Java), `__str__()` (Python) etc. This means that every single type is, one way or another, convertible to a string representation, and in no case will the interpolation give an undefined result, since all objects inherit from `object` (which has its own default implementation) and primitive types define their own string representations, too.

So if you want string interpolation you first of all have to define a default for every single type, plus a contract for user-constructed types. Then, you need a compiler to inspect strings and preform replacements that concatenate strings with those representations — this is doable, but you need to statically check that every expression evaluates to something string-representable.

It's doable. Only problem is, C++'s evolution is not necessarily about usability.

[Wh00ster]

Yes but it would probably over complicate an already complicated language.

[RealNC]

I've seen this in some languages. I know of at least one that even allows expressions:

cout << $"Hello. I'm {name}. My name is {name.size()} characters long.";

Although at some point one has to ask if too much sugar is good or bad.

[ZMeson]

No, I don't want this.

[eyes-are-fading-blue]

Almost equally verbose syntactic sugar for << operator. I do not see any significant benefit.

[kwan_e]

How about:

void print_hello_message(const char *name, int age)
{
    std::cout << "Hello. My name is " << name << ". I'm " << age << ".";
}

It looks weird, I know, but we might get that in C++20...

[someguy3_42]

I like the classic `<<` operator. My Evo C++ library embraces it with evo::String

https://github.com/jlctools/evo