You want to do:

template <typename T>
std::string numberToString(T number) {
    std::ostringstream ss;
    ss << number;
    return ss.str();
}

int main() {
    std::cout << numberToString(123);
    return 0;
}

To get the underlying std::string in std::ostringstream, and then the resulting c-style string in std::string. As others have pointed out, the pointer returned by c_str goes out of scope, and therefore you must copy it to a buffer or to another std::string. If you insist on using printf, then use c_str on the function call:

printf("%s", numberToString(123).c_str());

For more information, see Is it a good idea to return " const char * " from a function?

Depending on what somestlstring is and what is being done there.

If it is a local variable you are returning a pointer into memory that is being released when GetSomeString completes, so it is a dangling pointer and an error.

It all boils down to the lifetime of somestlstring and the operations you perform on it. The pointer returned by .c_str() is guaranteed to be valid only up to the next mutating operation in the string. So if something changes somestlstring from the call to .c_str() and before s is constructed you will be in undefined behavior land.

However, you can simply use std::to_string.

std::string s = std::to_string(123);

You want to do:

template <typename T>
std::string numberToString(T number) {
    std::ostringstream ss;
    ss << number;
    return ss.str();
}

int main() {
    std::cout << numberToString(123);
    return 0;
}

To get the underlying std::string in std::ostringstream, and then the resulting c-style string in std::string. As others have pointed out, the pointer returned by c_str goes out of scope, and therefore you must copy it to a buffer or to another std::string. If you insist on using printf, then use c_str on the function call:

printf("%s", numberToString(123).c_str());

For more information, see Is it a good idea to return " const char * " from a function?

Depending on what somestlstring is and what is being done there.

If it is a local variable you are returning a pointer into memory that is being released when GetSomeString completes, so it is a dangling pointer and an error.

It all boils down to the lifetime of somestlstring and the operations you perform on it. The pointer returned by .c_str() is guaranteed to be valid only up to the next mutating operation in the string. So if something changes somestlstring from the call to .c_str() and before s is constructed you will be in undefined behavior land.

However, you can simply use std::to_string.

std::string s = std::to_string(123);

You want to do:

template <typename T>
std::string numberToString(T number) {
    std::ostringstream ss;
    ss << number;
    std::string temp = ss.str().c_str();
    return temp;
}

int main() {
    std::cout << numberToString(123);
    return 0;
}

To get the underlying std::string in std::ostringstream, and then the resulting c-style string in std::string.

For more information, see Is it a good idea to return " const char * " from a function?

Depending on what somestlstring is and what is being done there.

If it is a local variable you are returning a pointer into memory that is being released when GetSomeString completes, so it is a dangling pointer and an error.

It all boils down to the lifetime of somestlstring and the operations you perform on it. The pointer returned by .c_str() is guaranteed to be valid only up to the next mutating operation in the string. So if something changes somestlstring from the call to .c_str() and before s is constructed you will be in undefined behavior land.

You want to do:

template <typename T>
std::string numberToString(T number) {
    std::ostringstream ss;
    ss << number;
    return ss.str();
}

int main() {
    std::cout << numberToString(123);
    return 0;
}

To get the underlying std::string in std::ostringstream, and then the resulting c-style string in std::string. As others have pointed out, the pointer returned by c_str goes out of scope, and therefore you must copy it to a buffer or to another std::string. If you insist on using printf, then use c_str on the function call:

printf("%s", numberToString(123).c_str());

For more information, see Is it a good idea to return " const char * " from a function?

Depending on what somestlstring is and what is being done there.

If it is a local variable you are returning a pointer into memory that is being released when GetSomeString completes, so it is a dangling pointer and an error.

It all boils down to the lifetime of somestlstring and the operations you perform on it. The pointer returned by .c_str() is guaranteed to be valid only up to the next mutating operation in the string. So if something changes somestlstring from the call to .c_str() and before s is constructed you will be in undefined behavior land.

However, you can simply use std::to_string.

std::string s = std::to_string(123);