std::string func(const std::string& str)
{
std::string tmp = ...
...
return func(tmp);
}

MZ>Кстати, этот девиз и на моём "гербе" тоже ...

SH>std::string func(const std::string& str)
SH>{
SH>std::string tmp = ...
SH>...
SH>return func(tmp);
SH>}
SH>

#include <iostream>
#include <string>

using namespace std;

std::string func123(const std::string& str, const std::string& acc = "")
{
    if(str.empty()) return acc;
    
    std::string tmp(str.c_str(), str.size() - 1);
    
    return func123(tmp, acc + str[str.size() - 1]);
}

int main()
{
    cout << func123("Test") << endl;
}

import std.stdio;

string func123(string str, string acc = "")
{
    if(str.length < 1) return acc;
    
    string tmp = str[0 .. str.length - 1];
    
    return func123(tmp, acc ~ str[str.length - 1]);
}

void main()
{
    writefln(func123("Test"));
}

    push    ebp
    mov    ebp, esp
    push    edi
    push    esi
    push    ebx
    sub    esp, 44
    mov    eax, DWORD PTR [ebp+8]
    mov    edx, DWORD PTR [ebp+12]
    mov    DWORD PTR [ebp-24], eax
    mov    DWORD PTR [ebp-20], edx
    mov    eax, DWORD PTR [ebp+16]
    mov    edx, DWORD PTR [ebp+20]
    jmp    L10
L12:
    mov    eax, DWORD PTR [ebp-24]
    mov    edi, DWORD PTR [ebp-20]
    mov    edx, 1
    sub    eax, 1
    mov    ebx, DWORD PTR [ebp-28]
    mov    ecx, DWORD PTR [ebp-32]
    mov    esi, eax
    add    eax, edi
    mov    DWORD PTR [esp+16], edx
    mov    DWORD PTR [esp+20], eax
    mov    DWORD PTR [esp+12], ebx
    mov    DWORD PTR [esp+8], ecx
    mov    DWORD PTR [esp+4], 2
    mov    DWORD PTR [esp], OFFSET FLAT:__D11TypeInfo_Aa6__initZ
    call    __d_arraycatnT
    mov    DWORD PTR [ebp-24], esi
    mov    DWORD PTR [ebp-20], edi
L10:
    cmp    DWORD PTR [ebp-24], 1
    mov    DWORD PTR [ebp-32], eax
    mov    DWORD PTR [ebp-28], edx
    jae    L12
    mov    eax, DWORD PTR [ebp-32]
    mov    edx, DWORD PTR [ebp-28]
    add    esp, 44
    pop    ebx
    pop    esi
    pop    edi
    pop    ebp
    ret