STL std::string not work in C++ builder XE10.2

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C++

STL

string

Builder

std::string f;
f.append(0,2);
f[0]=2;
f[1]=1;
//f="cd";

std::string s;
s.append(0,5);
s[0]=1;
s[1]=2;
s[2]=1;
s[3]=3;
s[4]=1;
//s="abcde";

unsigned int pp=0;
pp=s.find(f);

this code work fine in Visual Studio 2008 sp1,operation system is Win7 64Bit sp1

but in C++ Builder XE10.2,function "find" always return "npos"
if change strings with normal charactors,like "abcdef","cde",it's work well

How can I make std::string::find work fine in C++ Builder XE10.2 ???

What I have tried:

std::string f;
//f.append(0,2);
//f[0]=2;
//f[1]=1;
f="cd";

std::string s;
//s.append(0,5);
//s[0]=1;
//s[1]=2;
//s[2]=1;
//s[3]=3;
//s[4]=1;
s="abcde";

unsigned int pp=0;
pp=s.find(f);

change std::string with normal charactors work fine