Forcing String to int Function to Consume Entire String
Edit: In c++17 or later from_chars
is preferred. See here for more: https://topanswers.xyz/cplusplus?q=724#a839
For a given string str
there are several ways to accomplish this each with advantages and disadvantages. I've written a live example here: https://ideone.com/LO2Qnq and discuss each below:
strtol
As suggested here strtol
's out-parameter can be used to get the number of characters read. strtol
actually returns a long
not an int
so a cast is happening on the return.
char* size;
const int num = strtol(str.c_str(), &size, 10);
if(distance(str.c_str(), const_cast<const char*>(size)) == str.size()) {
cout << "strtol: " << num << endl;
} else {
cout << "strtol: error\n";
}
Note that this uses str.c_str()
to refer to the same string. c_str
Returns pointer to the underlying array serving as character storage not a temporary if you have C++11:
c_str()
anddata()
perform the same function
Also note that the pointer returned by c_str
will be valid between the strtol
and distance
calls unless:
- Passing a non-
const
reference to thestring
to any standard library function- Calling non-
const
member functions on thestring
, excludingoperator[]
,at()
,front()
,back()
,begin()
,rbegin()
,end()
andrend()
If you violate either of these cases you'll need to make a temporary copy of i
's underlying const char*
and perform the test on that.
sscanf
sscanf
can use %zn
to return the number of characters read which may be more intuitive than doing a pointer comparison. If base is important, sscanf
may not be a good choice. Unlike strtol
and stoi
which support bases 2 - 36, sscanf
provides specifiers for only octal (%o
), decimal (%d
), and hexadecimal (%x
).
size_t size;
int num;
if(sscanf(str.c_str(), "%d%zn", &num, &size) == 1 && size == str.size()) {
cout << "sscanf: " << num << endl;
} else {
cout << "sscanf: error\n";
}
stoi
As suggested here stoi
's output parameter works like sscanf
's %n
returning the number of characters read. In keeping with C++ this takes a string
and unlike the C implementations above stoi
throws an invalid_argument
if the first non-whitespace character is not considered a digit for the current base, and this unfortunately means that unlike the C implementations this must check for an error in both the try
and catch
blocks.
try {
size_t size;
const auto num = stoi(str, &size);
if(size == str.size()) {
cout << "stoi: " << num << endl;
} else {
throw invalid_argument("invalid stoi argument");
}
} catch(const invalid_argument& /*e*/) {
cout << "stoi: error\n";
}