如何检查const char *是否以特定字符串开头？ （C ++）

Question

I have a const char* variable and I want to check if it begins with a certain string.

For example:

string sentence = "Hello, world!";
string other = "Hello";
const char* c = sentence.c_str();

if(/*"c" begins with "other"*/)
{
    //Do something
}

How can I do this using the if statement?

Answer 1

To check whether a C string begins with a certain substring, you could use strncmp() .

For C++ strings, there is a std::string::compare() overload that accepts offsets and lengths.

Answer 2

You can use the c function strstr(string1, string2) which returns a pointer to the first occurence of string2 in string1. If the pointer returned is to string1 then string1 begins with what you wanted to match.

const char* str1 = "Hello World";
const char* ptr = strstr(str1, "Hello");
// -----
if(str1 == ptr)
  puts("Found");

Keep in mind that you're other variable will need to use it's .c_str() method in the context of the strstr function.

Answer 3

There are a few options that come to mind, one using legacy C calls and another two being more C++ specific.

If you really have a const char * , it's probably best to use the legacy C one but, since your sample code only creates a const char * from a std::string , I've offered other solutions as you seem to be working exclusively with strings as the true source of the data.

In C++, you can use string::compare or string::find thus, though the compare is likely to be more efficient since it only checks at the start of the string rather than checking everywhere and comparing the return value to zero (the find seems more concise so, if you value that and speed is not of the utmost importance, you can use it instead):

if (haystack.compare(0, needle.length(), needle) == 0)
if (haystack.find(needle) == 0)

Using the legacy C stuff, you can do:

if (strncmp (haystack.c_str(), needle.c_str(), needle.length()) == 0)

See the following complete program for an example:

#include <iostream>
#include <string>
#include <cstring>

int main (void) {
    std::string haystack = "xyzzy";
    std::string needle = "xy";
    std::string other = "99";

    if (haystack.compare(0, needle.length(), needle) == 0)
        std::cout << "xy found\n";
    else
        std::cout << "xy not found\n";

    if (haystack.compare(0, other.length(), other) == 0)
        std::cout << "xx found\n";
    else
        std::cout << "xx not found\n";

    return 0;
}

For the other options, simply make changes to the if statements shown above to match the samples given.