ES

ES: Expressions and statements

Expressions and statements are the lowest and most direct way of expressing actions and computation. Declarations in local scopes are statements.

ES.1: Prefer the standard library to other libraries and to "handcrafted code"

Code using a library can be much easier to write than code working directly with language features, much shorter, tend to be of a higher level of abstraction, and the library code is presumably already tested. The ISO C++ Standard Library is among the most widely known and best tested libraries. It is available as part of all C++ implementations.
Large parts of the standard library rely on dynamic allocation (free store). These parts, notably the containers but not the algorithms, are unsuitable for some hard-real-time and embedded applications. In such cases, consider providing/using similar facilities, e.g., a standard-library-style container implemented using a pool allocator.

ES.2: Prefer suitable abstractions to direct use of language features

A "suitable abstraction" (e.g., library or class) is closer to the application concepts than the bare language, leads to shorter and clearer code, and is likely to be better tested.

vector<string> read1(istream& is)   // good
{
    vector<string> res;
    for (string s; is >> s;)
        res.push_back(s);
    return res;
}

The more traditional and lower-level near-equivalent is longer, messier, harder to get right, and most likely slower:

char** read2(istream& is, int maxelem, int maxstring,
             int* nread) // bad: verbose and incomplete
{
    auto res = new char*[maxelem];
    int elemcount = 0;
    while (is && elemcount < maxelem) {
        auto s = new char[maxstring];
        is.read(s, maxstring);
        res[elemcount++] = s;
        // Once the checking for overflow and error handling
        // has been added that code gets quite messy, and there
        // is the problem remembering to delete the returned
        // pointer and the C-style strings that array contains....
    }
    nread = &elemcount;
    return res;
}

ES.3: Don't repeat yourself, avoid redundant code

Duplicated or otherwise redundant code obscures intent, makes it harder to understand the logic, and makes maintenance harder, among other problems. It often arises from cut-and-paste programming.
Use standard algorithms where appropriate, instead of writing some own implementation.
See also: SL.1, ES.11