将std :: string转换为const char *，获取错误

Question

#include <iostream>
#include <sstream>

template <typename T>
const char* numberToString(T number) {
    std::ostringstream ss;
    ss << number;
    return ss.c_str();
}

int main() {
    printf(numberToString(123));
    return 0;
}

My error:

1>d:\programming\euler\problem 4\problem 4\problem 4\source.cpp(8): error C2039: 'c_str' : is not a member of 'std::basic_ostringstream<char,std::char_traits<char>,std::allocator<char>>'
1>          d:\programming\euler\problem 4\problem 4\problem 4\source.cpp(26) : see reference to function template instantiation 'const char *numberToString<int>(T)' being compiled
1>          with
1>          [
1>              T=int
1>          ]

Why doesn't this work?

Answer 1

c_str is a member of std::string , not ostringstream . If you want to get a string out of the stream, use str() . Note, however, that returning a const char* from that string is wrong - the string will go out of scope before you can use the const char* . Therefore, have your function return a string (or have it get a buffer to write to):

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

Answer 2

c_str() does not exist for std::ostringstream . What you meant was:

template <typename T>
const char* numberToString(T number) 
{
    std::ostringstream ss;
    ss << number;
    return ss.str().c_str();
}

After you make that change, you will have another problem: you will be returning a pointer to a buffer that was just destroyed. To fix that, you should return a std::string :

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

Which you can do with std::to_string already, so it is really pointless to write your own function.

Answer 3

That's because c_str() is the member function of std::string which returns a const char* .

To get the underlying string of a strinstream, you must use str() .

The error was pretty much self explanatory:

error C2039: 'c_str' : is not a member of 'std::basic_ostringstream

Note however that you're returning a pointer to something (the underlying data of the temporary string returned by str() ) that will not exist after the return statement (ie in the calling code) and that manipulating that pointer will quite sureley end up in undefined behavior.

As you are in C++, you could rather return the std::string directly, and output it with

std::cout << numberToString(123);

which would be safer.

Answer 4

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);