Adding Multiple string values

Question

I have this function that gets 3 input(mostly LPCSTR) and combines them together and finally return the value:

template<typename T> inline T const& LPCombine(T const& lpStr1, T const& lpStr2, T const& lpStr3) { std::string str1, str2, str3, Combined; LPCSTR lpCombiend = ""; str1 = lpStr1; str2 = lpStr2; str3 = lpStr3; Combined = str1 + str2 + str3; lpCombiend = Combined.c_str(); return lpCombiend; }

If I print the value of lpCombined in the same function it's correct and strings or numbers are concatenated so well but If I print the returned value from the LPCombine function from another function like the main function the printed value is something weird and unreadable:

╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠╠

What is the problem?

@Chiel That isn’t good enough; often T would itself be a pointer type, which would have the same problem. — Daniel H
– Daniel H, Commented Sep 11, 2017 at 16:39
@Chiel The result of c_str is only guaranteed to last as long as Combined, so it gets deleted after the end of the function as Combined itself is being destroyed. — Daniel H
– Daniel H, Commented Sep 11, 2017 at 18:09

Unick · Accepted Answer · 2017-09-11 09:45:51Z

Combined is destroyed on exit from LPCombine function. As result, LPCombine returns pointer to freed memory and you have unexpected behavior. Maybe you need to return std::string from the function (use std::string as input parameters).

Yes, using a template for this is not a good idea in the first place, since you cannot implicitly cast just anything to a std::string or back.

Mr.C64 · Accepted Answer · 2017-09-11 13:36:54Z

When T == LPCSTR, i.e. const char*, your function returns const reference to a raw char pointer.

The main problem is that this pointer points to the memory of a std::string object (Combined) defined inside the function body:

std::string ..., Combined; ... lpCombiend = Combined.c_str(); return lpCombiend;

(Note that you seem to have a typo, lpCombiend should be lpCombined.)

When the function terminates, the Combined std::string object is destroyed, so the lpCombiend pointer points to garbage.

In particular, using Visual C++ in debug builds, uninitialized memory is marked with 0xCC byte sequences. Note that 0xCC is the ASCII code 204 = ╠(Box drawing character double line vertical and right), which is exactly what you have in your output.

So, you should consider returning a std::string object instead.

I'd also question the declaration and design of your template function. Does it really make sense to have const T& input string parameters? If T is e.g. PCWSTR (const wchar_t*), the str1 = lpStr1; assignment inside your function body won't work, as str1 is a std::string, and you can't create a std::string from a const wchar_t* raw pointer.

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