<iostream>: Wrong reading of float values

[Fedr]

Describe the bug

"1e-07" is read from std::istream as 1e-06

Command-line test case

#include <iostream>
#include <sstream>
using namespace std;

int main()
{
    istringstream istr("1e-07");
    double xx = 0;
    istr >> xx;
    cout << "xx = " << xx << endl;
    return 0;
}

STL version

Visual Studio 2022 17.7
Please use Runtime Library not from DLL

Additional context

First found here: https://stackoverflow.com/q/76937234/7325599
This is also DevCom-10450662 / VSO-1876474 / AB#1876474

[achabense]

STL/stl/inc/xlocnum

Lines 1022 to 1024 in 60f1885

	if (_Seendigit) {
	*_Ptr++ = '0'; // put one back
	}

It's eagerly adding '0' before making exponent part.

--
This branch is useless and should just get removed.

[Fedr]

Thanks. Could you please comment why DLL-runtimes of the same Visual Studio do not contain the bug? Is it because they were built from an older version of Microsoft STL?

[achabense]

Is it because they were built from an older version of Microsoft STL?

I guess so. There was an overhaul to the function in #3364 (merged several months ago); the branch might become redundant after the change.

[fsb4000]

Is it because they were built from an older version of Microsoft STL?

Fedor Chelnokov (@Fedr) Yes. Look at: https://github.com/microsoft/STL/wiki/Changelog#vc-redist-lockdown

To reduce the frequency of required reboots when updating VS (and VS Previews), we "lock down" the Visual C++ Redistributable containing the STL's msvcp140.dll. (Most of the STL is header-only, so most changes are unaffected.) Approximately twice a year, specific VS Previews will be "unlocked", so the VC Redist will contain updated DLLs. VS 2022 17.8 Preview 2 will be the next unlocked release.

[fsb4000]

Casey Carter (@CaseyCarter) , I found the same DevCom bug: https://developercommunity.visualstudio.com/t/v143-1437-istringstream-incorrectly-con/10450662 , so it should also be closed after merging the fix.

[CaseyCarter]

Thanks Igor Zhukov (@fsb4000), I've linked everything up.

This is a fairly nasty regression, we should consider backporting the fix to 17.7 / 17.8.