You may be thinking of [P1819](https://wg21.link/P1819): "Interpolated Literals".
------- Original Message -------
On Wednesday, December 28th, 2022 at 8:02 AM, Guillaume Racicot via Std-Proposals <std-proposals_at_[hidden]> wrote:

> There was a long time ago a proposal for format string, I can't remember the number. In any case, the old one should be rebased to be in line with fmt and if there's a new one it should also be based on fmt formatting.
>
> Also, if you truly propose the compiler replace f"aaa {expr} bbb" with "aaa " << expr << " bbb" it means that the following code would not work:
>
> void g(std::string);
> g(f"aaa{expr}bbb");
>
> This is because the bit shift operator will be called, but don't make sense in this context.
>
> If we based the string expressions on fmt, then the following should work.
>
> You might also have dangerous combinations if you use stream operators blindly:
>
> int a = 1;
> int b = 4;
>
> f"{a}{b}" // equivalent to "16"??
>
> There is also a couple of important questions to be answered:
>
> - What is the type of f""? Depending on the answer, using those can be as fast as format, slower or faster.
> - When is the expression inside the {} evaluated? Depending on the answer, you might there might have performance implications or a footgun with temporaries and scopes.
> - How are the format options sent to a f string?
> - Does using this language feature will require the inclusion of a specific header? The answer will probably depend on what you answered on the other questions.
>
> Em qua., 28 de dez. de 2022 03:59, Yuri via Std-Proposals <std-proposals_at_[hidden]> escreveu:
>
>> **********************************
>> ** Problem that f-strings solve **
>> **********************************
>>
>> String formatting in C++ requires more characters than minimally necessary.
>>
>> There are two major ways to format complex strings in C++:
>> 1. using operator<<, for example:
>> std::cout << "threshold is " << threshold << ", time is now " <<
>> time() << std::endl; (1)
>> 2. using std::format, for example:
>> std::cout << std::format("threshold is {}, time is now {}",
>> threshold, time()) << std::endl; (2)
>>
>> Both ways have problems: they require more characters than minimally
>> necessary to perform the task of formatting. The second way also separates
>> variables and their print locations, which makes it more error prone.
>>
>> It's common to need to do a lot of complex message printing for programs,
>> when various values or expressions need to be incorporated into the message.
>> C++ code in these cases is less expressive than it can be.
>>
>> **********************
>> ** Proposed feature **
>> **********************
>>
>> Introduce a special kind of string in C++: f-string.
>>
>> f-string begins with the 'f' character (f"string" or Lf"string"). When an
>> f-string is encountered it is interpreted in the same way as regular strings
>> with this exception:
>>
>>> The '{' character is assumed to begin an rvalue expression. Compiler
>> would
>>> change context after '{' and would begin to parse the rvalue, expecting
>>> to find the closing '}' after the rvalue ends. The closing '}' would
>> restore
>>> the string context. In order for '{' and '}' characters to appear in
>>> an f-string they should be escaped, \{ and \}.
>>
>> Compiler would replace all "...{<rvalue>}..." tokens in f-strings with
>> "..." << <rvalue> << "...", allowing them to be further processed in the
>> same
>> way as an operator<<-based print construct.
>>
>> This way forming complex strings in C++ would become simpler. The above
>> example
>> would look like:
>> std::cout << "threshold is {threshold}, time is now {time()}" <<
>> std::endl; (3)
>> This is more readable and intuitive than both (1) and (2).
>>
>> *****************
>> ** Limitations **
>> *****************
>>
>> (1) rvalues should be printable with operator<<.
>> (2) for simplicity macros wouldn't be allowed in rvalues embedded in
>> f-strings.
>>
>> *************************************
>> ** Affected elements of toolchains **
>> *************************************
>>
>> * C++ compiler
>> * Syntax highlighters in editors and other software
>>
>> **************
>> ** Benefits **
>> **************
>>
>> f-strings would simplify string formatting and allow to make string
>> formatting
>> code as simple as possible. This would make printing code less error
>> prone, and
>> would make it much easier to read.
>>
>> *****************************
>> ** What inspired this idea **
>> *****************************
>>
>> (1) While writing a lot of C++ code with many complex messages incorporating
>> a lot of various values I was thinking that it got to be a shorter and
>> easier way of doing this.
>> (2) While seeing f-strings in Python recently I realized that C++ can also
>> greatly benefit from this feature.
>>
>> *************
>> ** Example **
>> *************
>>
>> This code would be legal C++ code with f-strings:
>>
>> #include <chrono>
>> #include <ctime>
>> #include <iostream>
>> #include <pwd.h>
>> #include <unistd.h>
>>
>> using namespace std;
>>
>> int main() {
>> auto now =
>> chrono::system_clock::to_time_t(chrono::system_clock::now());
>> cout << "Hello {getpwuid(geteuid())->pw_name}"
>> ", the time now is {ctime(&now)}." << endl;
>> }
>>
>> Thank you,
>> Yuri Victorovich
>>
>> --
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