std :: string（empty char *）错误：basic_string :: _ S_construct null无效错误

Question

On Debian, I need to read an environment variable, otherwise return an empty string. Here is my code:

std::string getEnv(std::string var)
{
    char* val = ::getenv(var.c_str());
    std::string retVal(val);
    return retVal;
}

When the environment variable is not found, I'm getting the following error:

basic_string::_S_construct null not valid error

After debugging, the error code happens on std::string retVal(val) when I try to load an empty val to it.

I tought loading an string from an empty char * would work, but seens not to.

How can I fix the above code so that I can make it work in case the enviroment variable does not exists ?

Answer 1

From man getenv :

The getenv() function returns a pointer to the value in the environment, or NULL if there is no match.

So you need to account for the case where val == NULL .

std::string getEnv(std::string var)
{
    const char* val = ::getenv(var.c_str());
    if (val == nullptr)
        return std::string("");
    return std::string(val);
}

would be a possible solution (return empty string if var is not found).

Answer 2

you can use string::data() function to get the address of underling buffer pointer.

try var.c_str() or var.data().

  std::string getEnv(std::string var)
    {
        char* val = ::getenv(var.data());
        std::string retVal(val ? val : "");
        return retVal;
    }

For me its working.

val ? val : "" val ? val : "" to avoid passing NULL to std::string() constructor